I've never used plain water in my camelbak and I've never had any problems. Propel and Crystal light never left a flavor but Gatorade and Cytomax do, to some degree.

I do clean it out as soon as I get home and I have a cleaning kit (the nashbar brand rather than the official camelbak cleaning kit) for it so it's an easy job. Sometimes I fill it with water and add a denture tablet...sometimes I just use a tiny bit of dish soap and the brushes...and rinse really well. I always hang it to dry with the insert that keeps it open.

I guess a taste residue might be weird if you go back to water but if you stick with a flavored drink it's not a big deal.