Always,do the Chelsea chop. There are some plants you don't do it to....lupins, delphiniums, etc. the summer flowering things I cut back....the,Chelsea chop???..in mid summer. Things like argyranthemums and occasionally on cosmos where flowering may be less than its peak. A trim back and they flower again profusely to the autumn.
If sedums are cut back now the flowers should be just as big. I've experimented a bit with sedums and even had flowers on a very late cut back. This late cut produces fuller foliage but fewer flowers so now is good time