I use it all season, but only if the bike will be sitting for more than a week without being ridden. Call me parinoid, but I like to have the battery fully charged when I take off for a ride, especially if I plan on being any distance from home...
I just use mine occassionally ... typically just before an overnighter, just to be sure ... I ride just about every week, so I'm guessing that I really don't need it at all. I think if I lived where I didn't ride for a couple of weeks (or more), I'd have it plugged in all the time ...
If you get a tender make sure it is designed as one. Optimate Brand is a good standard to compare other ones to. Make sure it has a desulfating cycle built into the charging sequence... that really stretches the life of a battery, even if you ride very often.
Haven't installed the pig tail yet on my ultra, but have used one on all previous bikes for the past 10 years. Park it, plug it in and forget it. As others have said, it's cheap insurance. Also purchased the $15 model Walmart sells for my lawn mower. I keep it plugged in over the winter and the dry season when I'm not mowing regularly.