5 - always. You get about 20% more tire life across all 5. The spare wears at the same rate as the other 4, so there is no issue mounting one tire that is larger in diameter than the others. When the time comes to replace them, you won't have a 4-5 year or older spare tire that may have never been used. If you want to change to a different type of tire, you are not stuck with an odd-ball spare. You get practice using the spare mounting system, so if won't be a 'new' experience if you have to change a flat. The regular use of the spare mounting system, might keep it from rusting solid from non-use, and not working when you need it most.
PS: I use the 5 tire rotation method recommended by Bridgestone, not the one in the Toyota manual. I feel rotating tires from side to side, as well as front to back is better, and makes more sense, to me.