Just bought some second hand dt Swiss wheels and the valve size in rim are for Prestas am
I being foolish if I drill out for schreader size

well that’s all the rim manufacturer does to make a bigger hole. 9mm bit iirc

Why not fit presta tubes?

Usually 8.5mm, drilled lots of rims with no problems, but i’d avoid narrow section road type rims.

I drilled mine, regretted it slightly when I went tubeless as it was quite difficult to sort out schraeder valves…

^^^^ Exactly. Future proofing. I know Presta can be a bit fiddly on cold days with gloved fingers, but if the Good Lord meant us to be circumfornicating with punctures on the trail he wouldn’t have invented Stans jizzm.