I've cast down to 00wt rods, I really don't think there is a need to go that low, unless you can fford to have a rod just for creek fishing. I fish a 3wt and it's plenty fun and allows me to cast most flies and still zing out 70ft if needed. I would say that anything less is for small stream fishing only, I don' think I would go lower for most lakes.
Keep in mind though, in 4wt and under, there can be a huge difference in the abilities from one maker/model to the next. and line can play a huge difference. Th difference between a Sage LL rod versus a Scott G2 in the same weight is crazy.I would say for local trout/course fish fishing a 3-4wt is just fine. The action is a personal preference.