Is it OK to weld Tie Downs to Frame

I sure would not weld to the frame or drill holes in the frame for that manner. But, of the two, drilling is better as long as you use a hole sized for the fastener. In fact, drilling and tapping the hole for the bolt would be the best way to do this. By filling the hole with the bolt, most of the strength is restored from the drilling.

Many new trucks use a hydro formed frame. It's a hollow tube, formed with pressurized water, not an I-beam or U-channel. Drilling and through-bolting is bad too; big risk of over tightening and crushing/collapsing the frame member.