In my humble opinion, a fillet welded ladder rung is more efficient, as less labor is involved. It's also of sufficient strength. For example, one 3/16 fillet weld around a 3/4 diameter rung is good for 6,558#.

.928 (the multiplier) x 3 (3/16) = 2.784 (kips)

2.784 (strength of a one inch long 3/16 fillet weld) x 2.355 (circumference of the rod diameter) = 6.558

So, one inch of 3/16 fillet weld is good for 6,558#

It's never made sense to punch holes in a ladder stringer, which bows the stringer, then plug weld the rung, and and then grind it flush, when a fillet weld at each end will suffice.

I hear the excuse.... What if the fillet weld breaks? My response is always this... "You're worried about a fillet weld breaking on a ladder rung? If a fillet weld is good enough for a framing angle at the end of a beam, that's supporting a much, much heavier load, then it's good enough to hold a 250 # man climbing up and down a ladder."