The beat sheet is a tool. If the beat sheet is properly written--and it takes a lot of work to get it right--then I find that the script writes itself.
Some of my screenwriting students want to write their short film script first and then turn in their beat sheet. What reveals for me is is a lack of preparation. You don't yet know what to write.
If you find that while writing the draft you don't to follow the beat sheet, then you need to examine whether your beat sheet is correct. The beat sheet is a structural guideline, but it isn't meant to be how a story is dramatized, just WHAT happens. How it happens is what the draft is for. So if you're veering from the beat sheet, it may be only because you're getting into the creative flow of HOW something happens. But if you're veering from the beat sheet because your story is changing--ie WHAT happens is different, then you need to rethink your beat sheet first. Otherwise you will end up writing draft after draft.
You'll get there eventually, but one way is a lot more work than the other.