As long as it's not major, it shouldn't affect it. For example, I love using hyphens - I do it all the time. I don't know if it's grammatically correct - and who knows if it is, really - but... that just my -style-.
But doing thing's like putting apostrophe's in the wrong place's, or other things like that - then yes, you will lose a few marks. The reason is simple - you've had almost a year to write this thing, it should be very much close to perfect.
That said, I got a Band E4 in it - and sometimes I look at my work and I go "lol i can't believe that made it through", because either my wording was bad or I think once or twice, my grammar was off by a bit.