If you're going to use an HRM, you should calculate your max HR rather than use the theoretical calculations. There are a number of different ones around, some coming out pretty similar, others vastly different. I do HR training, know my max as that's what I got when doing the appropriate test, but if I went by some of the calculations, my max would be around 30bpm less than it actually is.
+/- 5 bpm probably won't make much difference, but +/-10 bpm would.
I take my RHR first thing in the morning, before I get up (or even move) luckily I've got a clock with a loud tick, so I can use that without even opening my eyes!