Putting in basement shower and do not want to use rough-in because of location (never put them where you want them). Want to punch a hole in floor to accommodate p-trap, this will put me 3-4" above the floor on a 'pedistal'? What kind of slope would I need so that all the water drains from shower floor? Also do I need to use a membrane, or being the basement is this necessary?

The standard slope is at least 1/4 inch downward slope per foot. I am assuming you have a drain pipe near the basement floor level. If not then you will need an ejector pump system.
I don't understand what you mean about cutting a hole in the floor though. If you are going to put the shower on a pedestal then you still need 1/4 in slope from the downstream side of the trap to the main drain.
I personally would use a membrane unless you are thinking of pouring a solid cement pedestal around the drain.

Was thinking of cutting a hole in the floor to accommodate the p-trap so that the shower floor would not be too high above rest of washroom floor, which would be tiled directly on top of cement basement floor. Think I have you about the membrane. If I'm right you mean if I use say plywood to build the pedistal then I should definitely use the membrane. If I use cement for the pedistal, don't need the membrane.

Slope: 1/4" for 2" pipe. You need to water proof bottom of your shower pan - even when it is in the basement. Its primary purpose is to drain condensation from the bottom of the pan and to the drain through weep holes. Condensation happens when your hot water hits cold concrete floor. It is not visible but it happens. Without water proofed floor it will be constantly wet, moist and mold will appear.

Water proofing will raise your floor by at least 2,5". Build "curb" around the perimeter of your new shower . Install drain with weep holes. Use either PVC liner of Hot Mop process to water proof. Cover with 2" of sloped concrete and tile.