What I would recommend is to take your own reel with the rated line for the rod and cast the fly shop rod with it. Line choice is individual.

Personally, I've never over or under lined a rod even the Sage RPLs that most folks thought were too stiff for the rated lines. Maybe I should have, but they seemed to work OK for me. As has been said before, the mass outside the rod tip varies with the distance you cast and that is constantly varying through out the day.

When you choose a fly line, you are really making a compromise about what you think your average cast is going to be. Most of the time, you will be casting below or above that average and it is unlikely that many of your casts will be spot on with the ideal. So realize that it is a compromise and not perfection that you are after.

What is really more important, I think, is the taper and the distribution of that mass. Choosing that correctly is more important in my view than getting the line wt spot on by 1/2 a line wt. Taper and mass distribution along that taper determines how that line will land and how it will carry your fly to its target regardless of the distance you are casting.