Short answer: stack spell hit/dmg (depending on hit capped or not).

Long answer: I don't believe there is a crit cap. I've never seen any compelling evidence, and abilities like shatter and combustion seem to imply not.

However, when looking at spell crit rating compared to spell hit and dmg, you'll see that it "costs" a lot more that those other two. This means you'll be getting less rating per item allocation. Also, crit takes more rating to equal 1% (22.1 rating to be exact), so it's even more skewed (compare to 12.6 for hit rating, almost twice as good).

This is just a rule of thumb btw, as gear, spec, level, and even play style will affect it. To really know one way or the other, you need to do some math; however, unless you are really min-maxing your character, you're fine just doing this.