The catch can on the dirty side should never be allowed to drain back to the crankcase. A lot of moisture can build up in the can and you don't want that in your crankcase.
They do have clean side catch can setups (i.e., the Camaro 1LE setup) that basically collected any oil that may have tried to blow back into the intake tube and catch it before it got back up into the intake (basically backflowing where it shouldn't). But that's only half the potential issue. In fact, unless you're pushing it a lot, you probably won't get much if anything in the tubing attached at the air inlet even without the clean side.
Recall, the LS series of engines were certified without catch cans of any kind, so it's totally up to you if you wish to run one. I do because it's not going to hurt, and it may be beneficial. I believe it is keeping more oil out of the intake system. Who wants that? The gunk you get out of your can when you clean it is gunk that would end up in the air intake side of your engine. If you're getting excessive blowby into the intake oiling up the air plenum, it's not a major deal if a little does depending on your fuel system. DI engines likely benefit further from the catch can on the dirty side than the intake injection above the valves. DI engines have nothing to backwash the valves, so periodic cleaning of the back sides of the intake valves will be in your future if you have a lot of oil sneaking into your intake system with the newer DI engines.
Remember too, even our stock "dinosaur" engines use a PCV that gets sucked into the intake side. A catch can can be used there too, IMO, is always beneficial and won't hurt a thing, unless you're needing to be factory stock. Any oil that gets sucked into the intake will come out the exhaust or will be stuck on the back side of the valves. You can minimize it or not. Your call.
When using a catch can, checking it often and emptying it before it gets full, is key. With a catch can, comes great responsibility. 🙂
