A little reinforcement, within reason, is a good idea. If for no other reason than to make you sleep better at night. Just don't go nuts with it. Also, just adding addition shear webs will be beneficial only if the spars are already strong enough to match the added webs. If not, spar doublers may be in order. It's like adding more concrete without beefing up the steel rebars. They must be matched in property for the additional bulk to make sense.

I've sometimes glued additional shear webs on the backside up to about 1/3 ~1/2 wing span. Just because the original designer spec'ed it one way doesn't mean that is the optimum setup. It is usually what he/she believes to be the ideal compromise between weight, strength and ease of construction, among other considerations.

One of the joys of building a kit, for me anyways, is the flexibility to alter the design a little. However, if you are not sure whether a particular mod is a good idea and how much strength it would add to the structure, given the weight penalty, it is better to leave well enough alone and build it stock.