you need a special tool from renault to fit the sealing ring properly (don't over tighten it) or the rubber seal will be pushed down into the tank.
and yes distortion can occur, i asked a renault mechanic at my local dealership.
you need the tank empty or near empty and it should be suspended or off the car ideally.
you may also have problems with the filler or its connecting hose, as i found we had a cracked filler after having to open the tank up to repair the tank level guage and have new straps fitted.
then i ran into problems with refitting and had to buy the special universal fitting tool (about £50 from renault).
if you are working on the tank, it might be good idea to consider replacing the tank straps if they are corroded. if you do, the tank MUST be fully supported and the filler hose MUST be released or lose so it doesnt get damaged by the weight of the tank on it.
torque on the renault tool should be no more than hand tight, no 2+ foot long rachet (its not a cylinder head bolt)
and using a screwdriver or chisel to open or close the ring is definatly a no-no(them pesky tabs tend to snap off).
A new ring and rubber seal are needed if you pull the pump/sender out, they do age and distort.