It is a matter of finding a comfortable position that 1) keeps the spine straight enough to squeeze mandible, maxilla and tongue together, 2) doesn't block airways. For many, sleeping on side encourages a head/pillow placement that simulates forward head posture and spinal sloughing. I'm not saying that you should stop sleeping on side, but that you should become more aware of your spine, because spinal position determines the resting position of both tongue and the jaws.

I did tape in the beginning, i'd do 2 diagonal tapes then one vertical in the centre of my lips. I also now sleep with as low a pillow as possible and on my back. It was tricky at first  but now I sleep without tape and my mouth stays shut all night.

Once you get into bed at night, get your tongue into the correct position, it will make it easier to wake up with your tongue in the correct position.

rogerramjet and I discuss our mouth taping practices in this thread:  /community/community/main-forum/how-to-avoid-mouth-breathing-at-night/paged/1/#

I feel that it is a matter of making sure that there is no nasal congestion before sleeping. You will only switch to mouth breathing (mouth opening) if it is difficult to breathe through nose. Assuming you can maintain correct posture throughout the day, it will stay during your sleep if you are not moth breathing.

Use nasal strips/spray or you could practice Buteyko method if congestion is a problem