I had to have my stem shortened to make my fit better but my LBS said as a guideline, you should stay within 20mm of whatever your stock was... I switched from 85 to a 65 They also said that the handling might get a little more twitchy as the stem gets shorter, and cautioned me about the handling at higher speeds. I don't go that fast or far (no more than 18 mph or 30 miles) so I'm not too concerned. If I build up my strength, I might have to change my stem again or something but we'll see.

I haven't taken my bike out since the change so I've only been on the trainer but I'm so happy about my fit that I haven't really given it any thought