It really only bothers me in public fora like hackerne.ws where your handle is already right in your face.
I don't mind it at all in basecamp which lacks structure or email where it's traditional and signing off is a way to make it friendly. That said, I don't use automatic signatures, I write a personal benediction/have-a-good-day type thing for each email.
If it's not worth signing personally, it's not worth writing the email.
The short rule is, if it's occurred to you that it might be douchey, you're probably safe because you actually care.
Think of it as a Dunning-Kruger for douchebags.
A...Douching-Kruger if you will.