I just had a heat pump installed. I had central air already so the duct work was there already for a/c. Should my vents in the ceiling be changed?
There is something to what you've heard, but it's not like you will get no heating with the current system. It may not be ideal, but it'll work. Unless you live in the Southern hemisphere, you should already know if it's going to be an issue or not. Most registers do not blow flat across the ceiling but at an outwards, downwards angle.

You could change out the registers for those with a more straight down configuration, or even a simple grille with no directional fins. But also consider what happens in the Summer. Air con blowing straight down is not good for the same reason it's good for heating. You should select the dispersion pattern for the most critical condition.

In primarily hot climates, air con takes priority, do what's best for that. It's the other way around for primarily cold climates. Or obtain adjustable registers so you can direct the flow as needed.