My doctor wanted me to get a mouth and nose mask. My CPAP technician convinced me just to get a nose mask. He said everyone thinks they need to breathe out of their mouth, but he suggested I would quickly get used to sleeping with my mouth shut. I was doubtful, but he was right. What you're doing is very beneficial. The only thing you're not getting, I guess, is the air being forced into your nostrils. You might not need that anyway. 

siggy47

HealthAndFitness

First Checkup After Two Months On My CPAP

I often tape my mouth at night. It forces me to breathe with the nose, which is highly desirable, and is effective against snoring and sleep apnoea.
My wife was initially sceptical, but she's now enjoying quieter nights :-) 