Sure, but in the case of grunge, the categorization has always felt pretty superficial to me. (Mainly it's about location... and flannel shirts, heh.)
Maybe I feel this way also because I found and fell in love with Soundgarden just before they started to talk about a thing called grunge. I heard "Jesus Christ Pose" on MTV Europe's Headbanger's Ball in 1991 (mid/late summer? Early autumn?) and was totally blown away by it. Actually, I do vaguely remember the host Vanessa Warwick mentioning something about Seattle's "lively" (?) music scene..
Yeah, the musical connections between the grunge bands were tenuous.. it was more about location.
Bundling them all as 'Seattle' bands was "superficial", but it was effective. And since a lot of 'em were coming up in the same clubs, it was an accurate characterization (as a legitimate local music scene).
Soundgarden was my first band from that scene also. But I heard 'em a year or two earlier.. they had a cool track on the "Pump up the Volume" soundtrack (if I'm remembering right).
If Soundgarden had come out of somewhere outside of Washington state (like Atlanta, or NYC, etc) they wouldn't have been viewed as grunge.. their location was critical to the way they were marketed/perceived.
My opinion, it is completely fair to debate whether grunge was an authentically linked music genre.. but ppl saying there was no grunge are lying, or trying to be too cool for labels. By the mid 9os, every teenager in America was aware of grunge= heavy rock, in flannel, from the Pacific Northwest. That awareness (no matter how misguided) was an objective reality.