Being assertive with your CFI, as awkward as it may feel with your inferior level of experience, may save your life. Especially with such critical items as fuel, oil, etc. For example, let's say you've been flying 10 minutes with the fuel needles buried in the red.
You're thinking, "Looks a little low--but those are inaccurate anyway, and he's a professional. I'm sure he's paying attention."
He's thinking anything from, "Only a few more months of this crap and I'm off to the regionals" to "Man, I'm starving; is it lunchtime yet?" to "Did I return those DVDs to Blockbuster?"
It's comforting to think of CFIs as machines who are at 110% of their game every day, but, you know, human beings and all that stuff. I don't think I can add anything more to the excellent advice given here by all so far, except that I've had one or two firsthand CFI-judgment-related experiences as a student that I would not care to repeat, so I feel I can't emphasize assertiveness enough.
MFR