You won't get it clean if you don't use some sort of detergent. As long as you don't get any inside the evap and make sure you rinse it off well what do you think it will hurt?

Changing expansion valve is a piece of cake .Seldom does it need to be done . If you have the slightest amount of air / moisture in the system it can cause an intermitant cooling/ warming cycle. A dirty evap can cause the same thing . A humid day, especially with a dirty evap can cause the same thing. Intenally or externally, it ices up from freezing humidity/ moisture, cloggs the evap till it thaws then repeats.Take it for what it's worth