Most glass rods have multi-line capability, some, like the classic Sceptre series, designed with that in mind, but many more because of the self-loading capability of the heavier blank. Personally, regardless of the size of the water, I would pick the line weight by the size of the flies to be used predominantly. Working back from the fly, then come the conditions and presentation needs. That determines the leader design and line weight combination. Then to conditions again, the length of the rod. I wouldn't hedge more than one line weight up or down from that, knowing that one or the other would work OK with the same rod if my initial judgment were off. But I would touch up the leader before changing line weights.
Stray up or down more than a one line weight and you may get outside the rod's best line handling and fish landing capability. Good idea to think about the fishing potential, not just castability. They may not match very well. Glass rods are pretty shock absorbent, but setting a 22 hook with a 7 weight will take more finesse than with a 4 weight. Same for landing the fish. Just one more thing to consider in deciding how to best use the multi-line weight capability.
Here's a good previous discussion. Although about a specific model, it applies to most midweights.
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