Ideally with metal TPMS valve stems you would use plastic caps. It's quite possible to have galvanic corrosion (two dissimilar metals touching) which would then fuse the two together. Probably not really an issue if you check tire pressure/fill them and remove the caps instead of not touching them for a year or so... The oem Toyota ones are a grayish plastic which honestly blend in with the wheels quite well and you never have to worry about the caps not coming off when you need them to. You can get them cheap on eBay or could probably talk to the service tech at a Toyota dealership and he's just toss you a couple...

-Nigel