Depends where. For a dome tweeter a dent can do a lot of bad things.
The use of a vacuum is a great fix. Also, with some tweeters the dome is removable and you can fix them by pressing from behind. This is the preferred solution with tweeters, as suction in a small area may be hard to control without excess force in the suspension.
Tweeters don't have a "spider" holding the dome in place. The surround is the only part of the suspension and it comes out when you disassemble the face plate, when manufactured to do so. That is, with the right screwdriver or Allen wrench the whole thing comes apart and gives you access to the back of the dome. Not true for mids and woofers I'm afraid.