I think that in 150K bike miles, I've had to replace headlights about a half dozen times on the road, maybe 1 tail light, maybe 1 turn signal light. I recently replaced my tour pack side lights and the fender tip lights with LEDs from superbrightleds.com. Amber wedge for the front, red for the rest, at about $5 ea.
Anyway, no light bulb is going to leave you stranded. Worse case is you ride to the next town on spotlights at night (done that too), pull the bulb at that auto parts store and get them to match it.
Although I could see carrying a spare headlight bulb, packing space is limited, so carry spares of stuff that will leave you stranded instead.
1986 FLHT: S&S carb, Andrews EV1
1987 XLH Hugger (Wife's): MegaSquirt Fuel Injection