Although I've never trained with one, I would have to agree with BTB. If you're working at around 90%, maybe a belt is a good idea. As your 90% numbers improve, so also will your lower (unbelted) numbers. There is no need to worry about losing strength in your midsection/core as long as your unbelted weights are going up.
Bogdan Petia Sarac - Must keel moose and squirrel
Cancer Survivor - 7/21/10
Benchmark 5K time: 27:45 (3/5/11)
It's not the weight we move, but the people we move that matters. -- Bearded Beast of Duloc (12/31/10)