Andrew,
I agree with BigB on this. I have pre-drilled and or rough shaped and sometimes it distorts the wood. My first thoughts was since the vacuum is equal all the way around there shouldn't be a problem. But after discussing with others and analyzing, it may the difference in wood density or moisture levels in sections of the wood. I really not sure but I had rather waste a little juice than loose the blank and the juice I put in it if it distorts. That being said, rounding off a blank a little oversize can safe you some juice and I haven't had a issue with doing that. Just some of my experiences.
Hope this helps,
Wayne