I personally think that a festival, or any long roadtrip is a great way to gauge the strength of any relationship --this includes friends. If you cannot get along for a festival or long journey...well I seriously doubt your going to be spending a large chunk of life together. Having said that, Davers makes a point that the opposite is true, that you can have a great time and still not work out.
My girl and I are now 6 years strong...we have with out a doubt fought at multiple times, about multiple things, at multiple festivals...but never left without being each others other.
For many people a festival can be a very straining point on a relationship. , If you are worried about going with your girlfriend, then go and find out. Going to a festival, especially sasquatch ,is rarely (never) the wrong answer.
Becuase if she/he leaves you after Sasquatch, the festival has actually done you a favour.
Honestly, the negative assosciation that I have with going to Sasquatch with significant others pretty much exclusively has to do with the fact that I evolve into a higher life form as soon as I get my tent set up. Its me, Im the bad dude. But am I a bad enough dude to save the president?