Cutting back is another way to encourage your perennials to produce healthy, new growth—and of course they’ll look tidier!
Summer pruning differs from pinching back…as it is done after the plant blooms instead. This is also different from deadheading because you're often taking off quite a bit more than just the spent flower heads.
To cut back perennials after blooming, use hand pruners to cut each stem to just above a leaf or bud. You will want to remove one-third to one-half of the stem's length. You can also cut back to reduce pest or disease issues; just trim off any damaged sections (all the way to the ground if you must) and the new growth will most often be healthy and free of the pests that plagued the old growth.
With specific regard to Salvia, keep in mind it will re-bloom again from the new shoots. Not as full, but will bloom again this season when you cut back and allow for new growth.
Learn more at this resource:
https://www.thespruce.com/pinch-cut-back-thin-perennial-plants-2539554