I don't have a clear answer for you. I was slightly overweight at diagnosis and am officially "not" now. My weight is still slowly declining so I hope it will stop when its enough or I may be in the same boat as yourself.
But here's a thought: we don't want to be too thin, but is it really good just to add some fat? Perhaps it would be better to exert effort to protect our lean muscle mass with careful attention to diet and add a regimen of regular resistance training to build muscle. It weighs more than fat anyway, so perhaps that's an all-around better way to keep weight from going "too low".