I use the dipstick oil temp gauge on my Ultra. Only useful when you're stopped since it's not easily or safely viewed while riding.
When I was researching info on my '09, most knowledgeable mechanics recommended installing an oil cooler on it. That and using synthetic oil. It really depends on how you ride, the temps you're riding in, are you doing alot of stop'n'go, high speed riding, etc.
It definitely doesn't hurt to have one and could help extend your drain intervals somewhat as your oil wouldn't get as beat up from excess heat.:cheers