If they are local bands, go to a show and meet them. It's easier to blow off a letter than a person and you'll have the opportunity to give them more information if they have questions. Offer to do it as an archiving service for them. If you already go to their shows and they play smaller clubs, they might recognize you as a regular fan.
Of course if they are likely to say no and you plan to stealth if they do, a letter is good.