I’m 6’4″ 36″ inseam and ride a 60cm. I couldn’t go smaller because my knees hit the bars when I was climbing out of the saddle. I thought I was gonna be looking at 62/64 until I test rode. Top tube is important as LS said, and this varies between brands. I certainly wouldn’t want to go larger on my Trek because it already looks like a gate!
Like cynic-al said it’s personal so see what you think when you’ve ridden a few sizes – try and get a good ride on each. Not sure what brands you’re looking at or where, but see what a knowledgable (ie not mountain biking saturday boy) staff member thinks, if they only have one size in to try don’t let them fob you off by saying it looks about right.