Surely they must have meaning? The key point is that you take the measurement under the same conditions. If you consisently take the measurement as the lowest point while sleeping, then you should get just as much info as when you take it at another reference point (e.g. in bed immediately upon waking up, or after sitting down for 15 minutes). The clincher is that that the key info you get (is RHR not elevated through overtraining or illness) will be available through all three reading methods(lowest HR during sleep, RHR upon waking, RHR after 15 mins sitting down).
For the purposes of finding out whether you're overtraining or not, all three will do, as long as you are consistent. Getting the nighttime/lowest HR during sleep reading seems to come with some hassle: you have to a HRM all night. So for general ease, stick with either RHR upon waking up in the morning or after sitting down for 15 minutes, I'd say.