Root hardy perennials are pruned every winter by the cold. Their tops die back to nothing, and they start each spring again from the ground. For these, the only reason to prune during the growing season is looks.
For example, Salvia will bloom big and hard late in the spring, and will then promptly spend energy ripening the seeds left by those flowers. However, if we prune those plants back to below the flowers as soon as they’re done blooming, we’ll usually end up with another (albeit slightly less robust) bloom about a month or so later.
With root hardy grasses, we generally leave them up all winter to add winter structure to the garden, but it’s important to cut them back to within several inches of the ground before they begin to grow in the spring. This isn’t for the health of the plant – it just looks better when the plants start shooting the green up.
There’s an important exception to the generalities here, and that’s the wonderful mums we love to see exploding with color in the fall. With mums, we should be pinching out the buds from the plant all the way up to the 4th of July. Maybe once a week, maybe every two weeks, but on a regular basis spend a little time with the mums as they grow to keep pinching those little buds off. This allows the plant to focus energy on the building of the plant itself during the first half of the summer. Then, when you stop pinching (or pruning) on the 4th of July, the plant shifts its energy from the green part of the plant into the making of the flower buds. The result is that the blooms all explode at about the same time in the fall – covering the plant in color – and the plant itself got to use much more energy to keep itself healthy and strong.