Definitely go with a knowledgable guide. Grizzlies are nothing to mess around with, and they always seem to appear when I've least expected them. The very second that I've been just a little complacent is when they've shown up to say hi.
And if you haven't encountered one before, nothing will truly prepare you for the experience. I've never been charged by one, but I was out hunting once with my cousin in-law and we crossed paths with a big one as we rounded a blind switchback. It stood up about 3/4 of the way and turned to look at us as we approached, and before I could even process what I was seeing it took off in a FULL run up hill through nasty brush and literally made its own f**ing trail. It could have easily turned the other way and killed either of us.
I'll go as far as to say that I hate grizzlies, but I respect the fact that they live there and I'm just a visitor to their habitat. I'd never hunt alone in their territory though.