Re: cutting back for rebloom and form ?
Some general rules for pinching and cutting back perennials: With plants like Salvias and Veronica that first bloom in the spring and tend to get more and more floppy as the summer progresses, an alternative to deadheading is to simply cut them back to a few inches above the ground (shave them), after the first flush of flowers. They will grow back with more compact growth and a new flush of flowers that will almost rival their initial bloom. A plus is it is a lot less time-consuming than deadheading. The down side to this method is if the plant is in a prominent spot in the bed you will be looking at the stubble until it grows back. Fall blooming perennials can be pinched back until mid July to make compact growth and summer blooming perennials can be pinched back in May, but it will probably delay flowering (which can be used to extend the bloom time if you cut back some, but not all of a particular plant.) Here is a good article on the Illinois extension website that goes into more details about pinching perennials.
Also check out this interesting article looking at cutting and pinching through the lens of botany.
In terms of the annuals, most will also respond well to a shave, as long as it is not too late in the season for them to recover. For instance, violas prefer cooler weather so if it is getting too hot for them even a shave won’t revive them. Petunias prefer hot weather, so don’t shave too close to the cold weather.