Here's some info on this subject you may find interesting. When I rebuilt my 292 motor summer before last, I installed a new Isky cam. I checked the center cam journal oil grove depth on the new cam and found it to be .018" deep. We put the cam in a lathe and cut the grove to .030" deep. Upon assembly, we checked the oil flow with the heads off by using a hand crank on the oil pump shaft. Oil immediately shot out of the holes in the block and we were satisfied that oil flow was adequate. On all my previous Y Block builds, I had always pressurized the rockers by closing up the oil drain tubes. With the tubes closed and the cam groove cut deeper, I had too much oil in the top end, To remedy the situation, I bought new oil drain tubes and installed them open in the stock location. This fixed the oil problem and my motor has run perfect since then. My valve covers don't leak and my rockers are well lubricated. I have about 2,000 miles on my new engine and when I checked the valve adjustment last weekend, they did not need adjusting.
I am pleased with the results of cutting the cam grove deeper. I think it's worth the trouble. In my opinion, much better than blocking off the oil drain tubes.