Just depends. If you are throwing the big weighted flies like Galloup and some of the new tiers like, then a bigger rod is useful. No reason why you cannot size down and still throw them with a 5 weight. Personally I hate throwing something as heavy as a 7 weight for just trout, unless they are REALLY big or perhaps very large water. A 6 weight is nice especially if throwing heavy heads along with bigger bugs. A 5 weight can work in moderate situations, but you really have to lighten up your patterns and rely on the line to get the right depth.