I run 1.3 to 1.5 depending on size of the tub, and I harem breed. Rarely do I have any issues with doing it this way, although since I don't control when the moms breed, sometimes I have a glut of babies and a dry spell. If you're freezing most of the produced rats to use later, you won't have that issue.

Don't put males together, don't handle pinks overly much with new moms. Listen for rats fighting, because occasionally two rats just plain don't like each other. And cull cull cull any biters, any fighters, any chewers... it's way easier to cull out undesirables than it is to deal with them. The only rat I've had to cull in months was a breeding female who decided to start beating up babies for no reason. She would grab one and stomp on it. So out with her. Otherwise, I don't have issues with my rats usually.

Keep them clean and fed on a good diet and never let them run out of water.