Yes, correct grammar matters. But we shouldn't let it stand in the way of a successful negotiation or let it aggravate those whose assistance we need.
I'm a good SNOOT: Syntax Nudniks Of Our Time, which was David Foster Wallace's fond nickname for extreme usage fanatics. David Foster Wallace, Consider the Lobster and Other Essays 66, 69 (2005). I'm one of those people who corrects—at least in my head—usage errors even in speech (just ask my children). But, after many years of ostentatious SNOOTness (those who are not SNOOTs will no doubt view it as SNOOTiness), I've learned the importance of ignoring many usage errors inserted into contracts by opposing counsel—or, more importantly, by my client—when the error poses no risk of causing a misinterpretation (or at least not one that would harm my client).
For example, many forms incorporate the following usage error: for good and valuable consideration, "the receipt and sufficiency of which is hereby acknowledged." We'll set aside Prof. Garner's call to strike this whole recital because it is "almost always unnecessary and unhelpful." After all, "almost always" is not good enough for risk-averse drafters. Bryan A. Garner, A Dictionary of Modern Legal Usage 738 (2d ed. 1995). The real question here is whether to correct the incorrect is by replacing it with the correct verb are. In your own documents and in documents that you have permission to edit freely (for example, those of inexperienced attorneys in your firm or those given to you for your SNOOT assistance), by all means make the correction. But in your client's form or your opponent's document, is this correction worth making? No. It has no substantive effect. In fact, the whole recital has no substantive effect in most cases. If an attorney changes is to are in someone else's document, that attorney merely displays proud and anal SNOOTitude. Instead, the diplomatic editor helps the client by avoiding meaningless changes and getting the deal done.
Of course, lawyers who care about logic will have trouble ignoring inexact language. Recently, a young lawyer in my firm came to me with the question whether to change the and to an or in the phrase "the lesser of (i) the amount of cash for that period and (ii) the amount required to cause total payments of the stipulated minimum rent" in the client's form document. He pointed out that or is clearly correct since it means one or the other, whereas and means both. He believed that and created a logical inconsistency. I told him that although he was correct technically (and mathematically), he was wrong politically. The meaning of the phrase was clear: either clearly means one or the other, and using and does not change this meaning, at least not enough to justify annoying the client.
The other side of tolerating an opponent's nonsubstantive usage errors is tolerating those who insist on editing that poor usage into your (precious) documents. For example, when another attorney replaces my perfectly good "this property" and "that property" with "such property" or said property I have to wonder whether folks in his part of the country describe a politician by saying "such candidate is a fool." But I generally leave the change untouched and feel superior.
People should always be careful when they make grammar or punctuation changes to others' documents without checking to be sure that their changes are correct. SNOOTs have long memories of grammatical slights. Years ago, when I was clerking for an appellate judge, another judge's clerk put quotation marks around all of the block quotes in an opinion that my judge had originated. The horror of it! Everyone knows that quotation marks are not used with quotes that are single spaced and indented! I was ready to go to battle over those quotation marks. My judge, however, being a sane person, an experienced politician, and the junior judge, told me to let it go, and he was right. Whether there are quotation marks around a block quote means nothing substantively. Those quotation marks did not justify a corrected draft. But I did lose respect for the other clerk—the one who had inserted the quotation marks—and I still remember her as the poster child for bad editing.
In most cases, correcting clerical errors and misspellings in others' documents is polite and proper and most attorneys are grateful for the catch. But if opposing counsel has had the temerity to edit a mistake into a SNOOT's document, then that SNOOT might be tempted to get even by leaving the mistake and highlighting it later with a strategic sic if the opportunity presents itself: "Your client has defaulted by failing to subordinate to the 'bolder [sic] of the security interests.'" This sic is rude, but the threat of it is certainly a reason to proofread your own documents and be judicious when editing the documents of others.
Of course, when a client has requested a full-SNOOT revision of an old form, you can tidy up organization and untangle sentences to your heart's content. In most situations, however, even SNOOTs should exercise restraint, that is, unless the quirk or mistake might create a harmful ambiguity. With tolerance, you'll get the deal done, feel like the literate person, and avoid having one of the more vindictive SNOOTs use that strategic sic at just the wrong moment.