Originally Posted by Doug64

I know what they’re for.
But used as that their function is entirely marginal.
What CAN happen is that a Presta tube in a Schräder rim begins to herniate into the rim around the valve stem.
If you want any significant benefit from it, it has to be on the side that sees pressure.
Keeping the valve stem centered and keeping water out - which is all you get from installing it in the hubside layer of a double wall rim - is quite unimportant.
I’ve used Prestas in Schräder drilled rims ”forever” w/o any grommets or inserts w/o any issues at all. I do cut the valve holes in the rim tape quite tight though.