be careful when starting to use nose clips
You need to have your glottis open when ascending with a nose clip. The air expanding within your mouth, sinuses and tubes will automatically equilibrate with your lungs. If you lock the glottis and keep your mouth firmly shut, I'm pretty sure that you could burst your ear drums (outwards), so be careful!
You can take off the clip but i really wouldnt recommend it. It can easily get wrapped around the line and get knotted up -I know this has happened to someone. Also you are just wasting air. In that case you close your gottis. But you waste the expanding air in your mouth/nose etc.
If you keep your clip on, and you pack before diving, be sure to exhale just a little in the last 1-2m before hitting the surface. Take care to time it just right - dont exhale too soon or too much. If you dont exhale your lungs will suddenly expand to almost maximum packing volume very quickly as you hit the surface- this is not good for the lungs.
alun