Body Language

Hello! I have been searching this site and others on dog behavior, and I have not found an answer to my question.

Lately, my 6 mo. old JRT boy has had the hair on his back stand up when ever he sees another dog (known or otherwise). His tail is still wagging, and he bows in a playful way, and he has never growled nor has he shown his teeth in an agressive way.

I had read that raised hair is usually a sign of agression...Is it in this case? He has been socialized since the beginning--Puppy Kindergarten (we start our next class in a few weeks), Doggy Daycare, Friends' dogs, the dog park, etc... Is this just a quirk of his, or is this something that I should try to correct? Sometimes his tail puffs out too--it all most looks like a furball on his rump! ha!