But I'm willing to bet you follow this blog because you're a writer, not a rapper, and if my hunch is correct, then grammar is definitely a problem.
Spell check and grammar check are our friends. But let's lump them in with the friends that tell you those pants don't make your butt look big when we know without a doubt we're dragging the Titanic behind us. While they make for nice acquaintances, they aren't always honest with you. To be fair, it's not their fault. They can't read minds and always know what we're trying to say (that would be pretty cool, though. Creepy. But cool). To make matters worse, our brain likes to play tricks on us when we reread, sometimes not catching the they're instead of their or won instead of one.
Will Agentawesome pass on a GREAT MS because of a grammar faux pas here and there? Probably not. But you want to make sure your work is as polished as it can be so that won't be an issue. If you've read over the MS so much it's beginning to look like word soup, give yourself a few days away and revisit. And always, always take advantage of valuable beta readers to catch those little mistakes that slip by.