Are hackles up always a sign of agression?

Tonight at the vets there was an extremely aggressive dog that was barking and lunging at everyone that walked through the door and then sat barking at all the other dogs all the time it was in the waiting room. When we walked in Reuben was interested in it and was pulling my OH to get near it but he never barked or growled and we just carried on and walked past. I noticed when we sat down that all Reubens hackles were up, although he did not seem interested once we had walked past it.
This is the first time he has ever done this and can honestly say he is a very friendly dog and has been well socialised and I was quite suprised to see his hackles up.

Do you think it is natural for him to react this way given how aggressive the other dogs was? He was fine with a labrador that came in later and they were having a good sniff (the nasty dog had left by this time) and normally Reuben would always want to play with any other dogs he sees.

No it's not always aggression. It can be excitement as well or fear as well.

or just showing the other dog he is not scared of him without getting violent

I always assumed the hackles went up to increase the perceived size of the dog. Same way a bird will puff out it's feathers. Not necessarily a sign of aggression, perhaps just a warning to back off.

yes i agree it can be out of fear