I overline my small stream rod. 7 wt line on a 4/5 wt rod. I also have a 5 wt spool for the reel and sometimes use that if I'm expecting to be casting more normal distances. On larger streams I generally just match the rating.
As Swattie said, it's not just a function of line weight and the rod, but also of distance. But there IS an ideal LOAD weight for each rod, err more accurately a range of load weights. The center of that ideal range should correspond approximately to 30 ft of fly line, past the tip, of the line wt the rod is rated for. A rod should have a decent "fluff" factor, though, meaning it's fine with 20 or 40 ft too, hence the "range".
But if you're casting really short distances, going with a larger line wt isn't overlining, it's properly loading. Likewise, if you're into super distance casting, going with a smaller line wt isn't underlining, it's properly loading. Each line wt is going to have a range of distances that it's appropriate for, and these will overlap.
For my small stream work I'm generally working with maybe an average of 10 ft of fly line beyond the tip. Note that's BEYOND THE TIP, and doesn't include leader length. If you do the straight math I probably should be throwin a 12 wt line on this 4/5 wt rod, as 10 ft of that is equivalent in weight to 30 ft of 5 wt line! That's not accounting for the taper or momentum effects, so it gets a bit more complicated. The reality is that, with a 5 wt line, it works but is not ideal in close, but is great from 20-40 ft. With a 7 wt line it's fine in close and can reach out a little when necessary, but over 25 ft or so it starts to break down. With a 9 wt line it's wonderful at around 10' but feels overloaded if you go much beyond 15', which is needed sometimes even on small streams. I settled in on a 7 wt line for small streams.