No rule - just ride what you find comfortable. Personally I wouldn't go swopping out too much at a time as there are so many variations you'd get lost amongst the choice, not to mention expense. Your LBS may look at how you sit on the bike and make suggestions as to where to go? That or try some different bikes fitted with this stuff.
Generally; play in the woods or prefer downhill duties, short stem wide bar. Longer rides, then go the other way - longer stem, narrower bar. One of the magazines this month has some guy fitting drops to his MTB..... It's your bike, do what you will. Lot of fashion out there that may not suit you.
MG