I have a Generation 5 Young Mametchi on my V3 right now.
I take pretty good care of it. I don't let it get over 1 or 2 lbs above the base weight and don't let it lose any hearts. I pause it at night though. That way I don't wake up to it beeping and throw it across the room and so it's not dead when I wake up.
It depends on how I'm feeling I think, I experiment on how I look after my tamas to see what everything leads to. Right now I have a Mametchi and a Bunbuntchi!