You should always advise ATC if you divert off an airway or off track, even if you stay within airway boundaries.
First, there might be some other traffic in the vicinity - maybe on an off-airway route - wich will be coming close to you, and your deviation could cause a proximity alert on the controllers radar screen, since minimum seperation could then be below limits. This is especially true in high density areas like all of central europe (I was given this info by an ATC controller).
Secondly, if you inform ATC, you keep them in the picture. Often ATC WX-radar is not as good as ours, and shows just the approximate position of a CB. Telling ATC about your diversion gives them the possibility to anticipate request for diversions by other aircraft, and they might start to vector everybody around the CB in general.
Just my thoughts...