If you're using normal caps, you can boil or sanitize. I wouldn't follow the advice of boiling them with the priming solution though, if only because the caps would get sugary and sticky.
If you're uncertain about whether to boil or sanitize, why not handle them the same way that you handled your bottles. After all, the beer is going to be in greater contact with the bottle than with the caps. What's good for the goose is good for the gander.
However, if you're using oxygen-absorbing caps (like these), then you shouldn't be getting them wet at all. Moisture renders the oxygen-absorbing liner useless. So do not sanitize o2-absorbing caps. Updated, see below.
Update: Northern Brewer Chris has provided some more information on the caps I linked to, so I'm going to update this answer. I found these directions on sanitizing o2-absorption caps from a company that sells them. Their directions boil down to:
- Pro-brewers don't bother to sanitize caps, so you can probably skip it.
- If you're not comfortable skipping it, then do not boil the o2-absorption caps.
- If you get an o2-absorption cap wet but don't use it right away, it's now useless.
- Don't use hydrogen peroxide sanitizers (One-Step, B-Bright) on o2-absorption caps.