If your shingles or gutters are damaged after you put a ladder on the roof, or worse yet after you HIRE someone to put a ladder on your roof, then there's something wrong. Obviously fixing the damage has been missed, but more specifically you're missing a ladder stablizer!
The image to the right shows what damage can be done to gutters when a ladder is placed directly on the gutters. Gutters aren't meant to hold the weight of a person, just the weight of the water that enters them! So naturally, putting a ladder against a gutter is almost certainly going to damage the gutter.
As an aside, you may also note the gutter guards here that are not installed correctly. Most gutter guards you can purchase at the local stores are not able to be angled specifically for your roof pitch, and they will end up pushing the shingles up and ramping water directly over the gutter. A double whammy if you will in this instance!
A ladder stablizer is a rather inexpensive addition to your ladder. It allows one to place the ladder on the roof without touching the gutter. It also allows you to put your ladder up at a window, and be supported by the walls on either side of the window. Here you can see how our ladder stabilizer allowed us to go above this same section of gutter without denting it more than the guy before us did!
Gutters are only supported by thin aluminum hangers. Formerly they were supported by large aluminum spikes and ferrules. But these are no longer used by most gutter professionals, and one of the reasons is the spikes put such large holes in the fascia they actually led to water damage in the fascia and subfascia!
As you can see from this picture, those hangers are not meant to support any weight at all above the weight of the gutter with water in it. Almost like canoe thwarts really, it keeps the gutter from collapsing inward, as the aluminum gutter is very flexible.
So if you're planning on getting on your roof, or if you're planning on hiring someone to get a ladder up to your gutter, make sure that a ladder stablizer is used. Then your gutter and shingles will stay in good shape!