Yes a quality doser helps, but even the highest quality doser will experience wear and tear in/on the dosing line around the dosing head. This wear and tear leads to cracks in the line that could lead to siphon one way or the other.
I would not put a check valve on the dosing line, that could even create undue stress on the dosing pump. The conversation about back siphon is way of the rails IMO. Don't put your dosing tube underwater in the sump and problem solved. If the fluid does go backwards (unlikely) it will just take air with it. Keep it simple, wipe the build up off the tip of the tube every few days or once a week and you should be good to go.
Well if the tube cracks, which is unlikely, it will such air and break siphon
Never in 20+ years of reefing have I ever used a check valve on a dosing line. Really don't see a need for it? If the line cracks, it's only going to suck air. Assuming of course your dosing line is above water?
ofcourse it is above water :)
