You need dihedral in a plane with a low, straight wing.

Roll coupling is caused by 3 things, wing location, high, mid, low;dihedral or anhedral; and sweep back of forward.

A low wing location, sweep forward and anhedral will cause a roll opposite to the rudder.
A high wing location, sweep back and dihedral will cause roll in the direction of the rudder.

So if you have a low wing, you need sweep back or dihedral to compensate for the location and cancel the roll coupling.

Conversely, a high wing plane needs anhedral of sweep forward to compensate for its wing location.

3 degrees on each side is a good rule of thumb for dihedral/anhedral. I don't build many swept wing planes, but, as I recall, sweep needs 5 times the amount as dihedral, so 15 degrees of sweep would compensate.

You only have to fly a plane with a low, flat wing and put in rudder to see what happens. In knife edge it will tend to roll over on its back. I have a couple of Ultra Sticks for sport flying. Both have 3 deg. anhedral on each side. The 40-sized one with an OS .55 will do a knife edge loop with correction only at the end.