I'd definitely discourage it asap.
I always emphasised to DD2 that she should always avoid leaving any unnecessary marks on the answer sheet, no matter how faint. I know very little about optical readers and 11+ answer sheets - coupled with that and the fact that I'm not a gambler, I felt safer that there was one less "what if?" worry to torture myself over.
I feel pretty sure that Patricia has some tips and strategies in the Bucks section, for quickly dealing with vocab you're not sure of, for synonyms, antonyms and compound words etc. As time is of the essence, we also taught DD2 to avoid erasing/rubbing out her answers on the questions sheet, unless she'd run out of "working-out" space
As Amber has said, some consortia don't allow you to write on, or mark the question sheet in any way - so I'd recommend checking out whether you can, for your area