I saw some questions and answers on air compressors so I thought I would ask one: I just purchased a Chicago Pneumatics 21 gallon compressor and I noticed a tiny dent about 3+ inches from the upper seam, (almost unnoticeable, like the edge of something hit it or the tank fell on it). Again, it is really shallow, but is this something to worry over?
As the reservoir for an air compressor holds air under pressure, the forces on the dent will be outward. If the dent is not on a seam or other metal discontinuity, and is shallow enough, little will happen. When it is pressurized the first time, if it does not "un-dent" itself, you likely have little about which to worry. Even if it does pop out, it won't go beyond flush and again, no worry.
Keep an eye on the dent. If the paint is scratched and the metal under begins to rust, not so good. If the paint is cracked or scratched, consider to apply primer and topcoat to remove future rust problems.
Rust is a problem over long periods of time, especially on the inside, so drain your tank frequently and examine it for outside rust and damage as well.