When roughing in a toilet in a cement slab do you need to leave a groove around the 4" sewer line to let the flange set tight to the cement floor?
Or do you make sure the foam cap is below the cement a little when pouring?

I usually wrap about four twists of sill seal around the riser, which leaves me about 3/8 of an inch of play when removed after the concrete has been poured and cured.

I usually use a 3" pipe to get to the toilet and a 4" riser and cap it exactly at floor height so the concrete man can skree over my pipe and give me a flat floor then when the time comes I tap out the cap and use a 4" x 3" floor flange and glue it inside.

When roughing in a toilet in a cement slab do you need to leave a groove around the 4" sewer line to let the flange set tight to the cement floor?
Or do you make sure the foam cap is below the cement a little when pouring?

In the Tampa Bay area we use a styrofoam closet wrapper that we duct tape around the raiser before the floor's poured. This is simply a piece of styrofoam 4 X 8" around and 3/4" thick that prevents the cement from burying the raiser. Then we just knock out the skim and dig out the styrofoam where we can glue in a regular closet flange. We do not use inside flanges on a slab unless the plumber messed up and forget the wrapper.
Good luck, Tom