No, although you should try to be as consistent as possible. In general, waking temperatures tend to creep up about two-tenths of a degree for every extra hour you sleep in. Thus, if you take it substantially later than usual, it may result in a reading that is outside the range of your usual pattern. If you wake up earlier than usual, you should take your temperature upon awakening, so long as you have had at least 3 hours of consecutive sleep.

Regardless, an occasional aberrant temperature can easily be dealt with by following the Rule of Thumb. You should also be aware that if taking your temperature feels like a burden, you can in fact take it for only about a third of the cycle without sacrificing contraceptive efficacy, as described in Chapter 10 of Taking Charge of Your Fertility.