1- Yes- pinch it back- take out the growing tip--that's called "topping" & will result in a bushier plant with more laterals. This is a "good thing". They will still reach their full height. When you plant out you can plant deeper than they are in the pot to give them exrta stability (like you can do with tomatoes). Be sure to pound in a stake when you plant so you don't damage the tuber putting it in later.
2- yes there is an advantage especially with the AA & A sized dahlias. One good thick sturdy stem as opposed to 2 not so sturdy stems is the result. You can let the laterals grow from this stem & you'll get bigger blooms & more of them. They're also easier to tie up if there's just one stem & it's laterals instead of 2 to get in each other's way. Go to the Colorado Dahlia Society site & look up the part on topping, debranching & disbudding- It's under Dahlia Care & Culture-- lots of good info & pictures to help you along.
If you want you can make a cutting out of the pinched off stem & get another plant for your money!

Here is a link that might be useful: Colorado Dahlia Society Site