I think NEC specifies a sustainable 5% tolerance from 120v, (114-126v) with up to a 13% drop or 6% overage for limited durations. If you really wanted to add a fixed voltage drop, get a nice extension cord (at least 14 gauge for a high amp roaster, preferably 12 or 10) A 50ft 14g extension cord will probably take 3-5 volts off under load, 2-3v on a 12g, 1-2v on a 10g. This energy is going into heating the wire, so don't get carried away. The real way to control voltage is with a transformer, like a variac (variable autotransformer)
Tex, there are a lot of things that could cause different readings at the outlets, load on the circuit and/or leg, voltage on the return, length of the run, gauge (or material) of the wire, measuring at different moments, etc. Polarity isn't really one of them.