How often do you have power outages and for how long? Without a crystal ball this is not entirely predictable but there may be trends and a typical service outage repair time associated with your area.
If power is not out very often the need for the heater is reduced, time (years) instead of cold start cycles will be what takes its toll on the generator. While it is a generator not a car, there is a similarity. You'd be best off with a lower viscosity oil in the winter than summer. If you're really *budget* minded and don't get enough hours on it to throw out the oil, you could even put the summer used oil back in the next summer, winter back in next winter. This assumes it is outdoors, not plumbed/ducted outside and located indoors since you mentioned the winter temperatures?
What is the value of this generator? $100 to reduce wear on an expensive one is a better investment than on a less expensive generator, plus constantly heating your oil just in case the generator is needed, is a constant power bill increase. Calculate the heater wattage vs your annual cost to run it multiplied by # of years (run hours) expected lifespan.
I'm mentioning all this because it is true that it isn't absolutely necessary with 5W-30 oil in your climate. I would lean towards no, you will be financially better off without it, but if there is some circumstance were a frail person could have a severe medical hardship were the power to go out for too long (even if just powering a modest indoor/room heater in winter), it may be worth the money to get the oil heater just in case.
