I recommend to take some key factors into account BEFORE buying a fly rod:
1. What flies will you present to the fish?
2. At what distances will you have to present your flies?
3. Will there be heavy winds often?
4. How spooky will the fish be?
5. How strong will the fish fight?
Based on the answers to these key questions I recommend to chose a fly line supporting your fly fishing situation best - again BEFORE you buy the fly rod.
1. Heavy flies = additional line weight will help to cast the flies more easily.
2. Long casts = additional line weight will help to achieve more distance.
3. Heavy winds = additional line weight helps to get less blown out of direction.
4. Spooky fish = less line weight will support to not spook the fish.
5. Fishing for smaller to medium sized fish you may prefer to have a fly rod being pretty smooth instead of a pretty stiff one.
Let's assume you have find an appropriate fly line matching to present your flies just perfect to the fish.
What fly rod would you chose now?
One that matches in it's line weight recommendation for your fly line or would you up- or down-line?
I think it mostly is smart to look for a fly rod being designed for your kind of fishing and even better your specific fishing situation (if available). Here the line weight recommendation labeled on the fly rod should match your line in order to get a great supporting tackle adjustment in your fishing. Most fly rod designers have a significant knowledge and label the line weight recommendations on their rods for proper reasons!
Let's have a look at an example for up-lining:
Coastal Sea trout fishing: I want to cast flies weighing upto 1,5 gramm. To cast these flies I prefer to have a 9wt. fly line. Most fly rods being recommended to be used with a 9wt. fly line are pretty heavy in weight when casting 8 hours a day for max distance. So I choose one being recommended for 8wt. fly lines. That rod comes in less weight making it easier to cast all day long. It also matches better for playing the average sized Sea trout of 45-55cm. I don't like any stiffer rods for playing these fish.
... and for matching the line and rod labels:
Now if I would want to cast only small flies around 0,5 gramm I would still use the same fly rod but hang on an 8wt. fly line instead. So for the smaller flies I would not up-line anymore.
... and for down-lining:
If in addition to fish small flies the conditions are calm I may even decide to down-line to a 7wt. fly line because this would help to spook less fish. In calm conditions a 9wt. fly line helps to spook much more fish as it happens with the 7wt. fly line. Of course using the 7wt. fly line will make for less rod bend during casting (it provides less resistance to your fly rod compared to the 9wt. fly line). Thus your rod now will feel more stiff. Playing the fish it still will offer (nearly) the same feeling.
Let's have a look at another fishing situation:
You are fishing for a pretty spooky (small to medium sized) fish and want to cast small to medium sized flies. All that will happen on short distances on a small stream with mostly no room for back casts.
I recommend to use a fly rod typically being recommended for 3wt. fly lines. Now I would hang on a 5wt. fly line. This outfit supports casting small to medium flies and not too easily spooking the fish as it may happen with a 6wt. fly line for example. Still I could handle a bit of wind coming into play sometimes. When mainly only roll casting I would want to create some significant rod bend matching wider casting arcs (10/2 instead of 11/1) for hitting the desired line speed best. In my expereinece most casters get better results and feel better with a quite soft rod (in relation to the line weight) when roll casting short distances. The soft rod properly matches to fight the smaller fish in addition.
A lot of the (internet based) discussions about up-lining (again using a heavier fly line as the rod designer labeled/recommended on the fly rod) recommend to up-line because you get more rod load. And loading the rod deeper makes for better casting (they say). It was often said the beginners would get the same amount of load with the heavier (up-lined) fly line as the experts get it with the one matching the label. I recommend not to think about rod loading at all and instead focus on all aspects I offered above. By the way there is a term "over-loading" being included to many of these discussions. Over-loading to me is breaking the fly rod based on having chosen a too heavy fly line. I yet have never seen this happening to anyone. What happens is that casters can't control the amount of rod bend when up-lining too much at some point anymore. Usually I never see this happen when up-lining one or two line weights. So yes, there is usually room for a given rod to up-line in order to transport heavy flies or handle wind much easier.
For further aspects you may join a proper discussion right here on the Sexyloops board.
Great week to all of you!
All my best
Mainly fly casting and teaching it during the last days.