@Colin Reilly Having grown up in Dover and my husband in newmarket I can tell you, you will find great tenants in those towns and also bad tenants in any of those towns. There are streets in both dover and Newmarket to steer clear from, but overall they are definitely nicer towns just prepare to adjust your expectations for cash flow (margins will be slimmer).
We have had great tenants and our current tenants are three young adults out of college who are all teachers and can't afford the expensive rents those other towns have. They are quiet, pay their rent early, and are ideal tenants. We screened our tenants THOROUGHLY to prevent bad tenants (as much as possible), but we definitely get all sorts of crazy applications and calls from unfit tenants. We pre-screen over the phone with our criteria before we ever show the apartment to avoid wasting our time and to weed out any potential unfit tenants.
Good luck in your search! If you are set on those towns you might want to get creative and try to find some deals off the market, or have all your ducks in a row to be ready to jump as soon as you see a potential deal hit the MLS!