That's a matter of choice. Not really necessary, but there are advantages (and disadvantages):
1) Will ensure a fresh spare forever ("shouldn't" make much difference until decades later.
2) Prolongs the time period slightly (25% more) between new tire purchase requirements.
3) Removes limits on driving with spare (speed etc.) if you need to actually put it on the truck & use it
1) Takes up more space in the trunk. (not a big deal)
2) Costs more (likely 25% more) to replace a 'set' of tires.
3) Requires more time/effort to rotate tires.
4) Requires an additional TPMS sensor to be bought/maintained.
5) Complicates the "winter tires" equation for those with more than one set.
6) You may actually over-extend the life of a 'set' of tires beyond point where "age-wear" is more critical than "miles-wear". This is for those that put very few miles per year on the vehicle.
7) You CANNOT use rotation-direction-specific tread types, without losing the benefit of having the 5th tire (it can only be used on one side, so even rotation among all 4 tires is impossible).
Personally, I can't really think of a 'justifiable' reason to do it, unless you've got specific road/region related peculiarities that don't allow the smaller spare; but otherwise it's a personal thing. Back in the bad old days, it made sense, due to more critical wear factors for tires. They just didn't last as long for various reasons, and moving the tires around could help even out otherwise uneven wear, just generally prolong the life of a set. That problem, which used to be fairly commonplace, has been pretty much eradicated by technology (improved tire, wheel, vehicle, & balance/alignment equipment) over the decades. If your vehicle & tires are properly maintained, you should be fine just rotating front to back.
I guess if you have a tire that's damaged beyond repair you have a tire with similar mileage on it that you could put in the rotation, however the new spare would be at full diameter.
Currently I'd use the donut spare, however I've purchased a FS rim and do want to purchase a FS spare. While I haven't done so, I'm looking for a FS tire either new or slightly used to put on the rim. I'll keep it in the trunk till needed.
If you don't tow, then I would not get one. If you tow, like I do, then I would get a full size spare, as the manual recommends, and put it into rotation. One benefit from putting your spare into rotation is that it helps to keep the rubber in good shape, it's less likely to dry rot and it forces you to keep your spare in good condition so when you need it it's "good to go."
Also, a full size spare doesn't take up that much space for it fits in the same storage location as the donut. You just have to relocate the foam tool box to the right side storage position above the muffler, which it was designed to support. I find that to be a pretty cool bit of engineering design; good job Gary Flint. :act035:
i wouldn't worry about a slight height mismatch in tires. all tires spin at slightly different speeds during normal driving unless traveling in a completely straight line. even when the traction control kicks in and sends power to the rear wheels they are not locked together. mismatched height is only an issue if you use vtm lock on hard surfaces. as for putting the full size spare in the trunk while towing consider that you will have to lift that tire out with the tailgate down and not swung to the side. I use the spare tire mount in the bed for my full size spare while towing. then again i am a little guy, 5'6".