Not every curly tail is made to rig tail down. Some with a body shape that puts the belly down have a tail up design.You can't put the tail down without rigging the body wrong.Some of the swimming minnows are like that.Also the size and profile of the tail will determine if it gets hung on the hook.Some baits can be rigged tail up and never get on the hook.Some of my most productive minnow and shad body grubs are designed to be rigged tail up,and have a large tail that does get on the hook at times....but if it catches fish like crazy i don't care I just clear it when it gets hung.Some tails will swim better and impart action to the body also when rigged one way or another.The tail will swim differently and the fish will have a preference many times,if it makes the body wobble or roll it will usually catch fish better. Try whatever and don't get hung up on one way or another just do whatever works best. I have caught just as many or more fish on jigs that are designed for tail up,and i have caught just as many or more fish on straight body grubs rigged up as down.