If you stay on good highways I wouldn't see a problem. If you have AAA then maybe forgo the spare, but if you aren't packing lots in the trunk and need the tire well space, then better safe than sorry. Make sure you have lugnuts that will work on your spare if need be. I don't know what your 18s are but some have lugnuts that don't have the bevel to work on the temp spare. Maybe ask in wheels and tires too. They could tell you about any heat issues, but I wouldn't think there would be.
