Ditto for most of the advice above.
I think the biggest thing is that you want your volley to skid and stay low. Backspin will help with that. You want to force your opponent to hit up to you at the net. You don't want the ball to sit up.
Also what would be the point? You're a couple of feet from the net. How much margin are you going to gain with topspin?
Practically you will have a very hard time (like impossible) putting topspin on a ball that's ripped to you at the net. Sometimes you're happy just to get your racquet in front of it. A lot of volleying is about blocking and angles. You really have to have one grip for both fh and bh because of all of this. Continental is the best answer, and that that grip isn't great for topspin even if you had enough time to generate a swing upward, which you don't.