If you want to do it go for it. Auto manufacturers will tell you for the life but then add asterisk for only under "normal conditions" It mostly to get the cost of ownership low on paper because some people really do shop for cars by looking at that stuff.
If its just a spare for me I really don't care about any recommended wheel widths to fit that tire. If you were planning on rotating in the spare then yeah I would care more but its just a spare.