I personally find that I need to stay with bars no wider than my shoulders - 40 - otherwise I get upper back/shoulder soreness. I swapped out the stock Bontrager that came with my Madone for a FSA model, suggested by a bike fitter, that measures 40 but the drops flare in ever so slightly making the tops a 40 but the drops more like 39.5, and this has been very comfy for me.
2001 Trek 7500 FX, converted to a hauler - Serfas
200? Marin hybrid - Selle San Marco
2004 Trek 5200 - Avatar
2011 Trek 6.2 Madone - Ruby