If you crack open the system to a point of changing a component, it's never a bad idea to change the dryer. Do you have a way to evacuate the system? If you have a way to evacuate the system, and you have replaced the filter dryer recently, you might get by with it.
If this is a 134 system, and you can pull a vacuam for 24 hours it wouldn't hurt.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in