I've only ever found out so can't say how it might be different not to. I think I'd still bond if we left it as a surprise but we prefer to know what we're having. We start calling bubs by their name, this time we've been able to talk to Moo about his little brother, we talk about our future son in general terms to each other etc. But yeah I think it helps us to bond

If you decide to not find out at your 3D scan - tell the person before they start. I'm sure they'll ask you anyway but when we had our last one done, the first thing we saw was a big pair of boy parts even we could tell what we were looking at on that big TV! Thankfully we already knew!