I try to use a double-overhand grip for warm-up sets and a mixed grip for the work sets. My last peak workout involved warm-up sets upto 245 x 5, and this seems almost like the limit for me at this stage with a double overhand grip.
However, this is not to say you cant deadlift heavy loads with double-overhand grip. I couldn't, but here's someone who did (700 lbs, no less!!) -