Your reading will be as exact as possible (depending on your meter) if you connect to the battery, however, (here comes the long answer) If you take it from a nearby light and that circuit is not drawing any power, (the lights and anything else on that circuit are all off) The reading will be the same as on the battery. If lights etc are on, then the resistance of the wire and the current being drawn will cause a small voltage drop. You can calculate the drop using Ohms law.
I wired mine into the circuit that powers the propane detector. The current drawn is small and is constant so that was good enough for me.
Be aware that voltmeters are not exact but are made to a certain tolerance. Depending on the accuracy of your meter you could be as much as 10% off, but probably better. IMHO, it is more important to be off by a constant amount than it is to be exact.
