Don't cut it off at the firewall.

If you do run it all the way to the coupler, you'll soon see that the coupler won't move in or out. I'm guessing you really meant, is it run as far out as possible without restricting the throttle movement. Yes, but with just a little extra room.

The outer tube helps the inner tube or flex cable to stay straight and keep from flexing. When the cable/tube that's supposed to do the work flexes, the flex robs the movement that's supposed to go to the throttle. So you run the outer tube as close to the servo and as close to the throttle arm as you can. You also glue the outer tube in lots of places along it's path to keep it from moving. If the whole thing flexes, it robs the throttle movement as well.

Getting the outer tube close to the servo and throttle is an art. You want to be as close as possible, but you don't want the outer tube to fight the natural flex that's going to be required by the circular path the servo arm and the throttle arms must follow. Some outer tubes are really flexible and they need to be supported every inch of the way. That is, glue them to something about every inch.

Some people actually glue a brace to the firewall that extends out to hold the outer tube so it won't flex too much. And when using flex cable, some people run hard solder into the last inch or so of cable that's unsupported by outer tube at both ends. All that could be considered overkill, but it's not wasted effort if you have the time and inclination to do it.