To add to the general collection of thoughts here, while the typical weight of the catch may set a sensible minimum on rod weight due to the correlation between the weight of line it's designed to cast translating into lifting power...but other than that very general quideline, the primary determining factor on what weight(s) are appropriate is simply what you plan on casting.
I switched from 5wt to a combination of a 4wt and a 6wt for most of my trout fishing...6wt sees water any time there's even a chance I might want to throw bigger streamers (3"+) or run any kind of a "rig" (indicator, tandem nymphs, dry-dropper, etc.). It handles the dries just fine too, don't get me wrong, but if I know it'll be a day of dry fly fishing, and no streamer bigger than a #8, I'll go with the 4wt.
It's also worth noting that a good fast 9' 6wt is ideal for not only streamers & nymphs for trout, but also smallmouth and Erie steel. So if you target smallmouth or hit the PA tribs at all, this rod sees even more use.
Personally, it's what I use for 80% of my summer fishing, as I fish a lot of water where there are just as many smallmouth as trout, so it's no big deal for me to be fishing a #14 caddis dry only to find a deep hole and re-rig to work the bottom with a #4 crawfish pattern or a #2 clouser...then go right back to the #14 dries for the next riffle.
I've never met a configuration that can handle that better than a fast 9' 6wt.