When I was initially gaining weight, I stopped all cardio as I would lose weight too easily. Now that I've been holding my current weight (265ish) for some time, I have re-introduced quite a bit of cardio, and it's not subtracting from my strength gains at all. I actually think it helps my recovery. I don't do a ton of it, I keep it moderate, and the health benefits are undeniable. If I were to do it all over again, I'd keep some easy cardio in there to stay in decent shape as I went up. If you're slowing down in your weight gain progress, simply eat a little more.