Staying still is useful.
HOWEVER not if it become uncomfortable or painful.

Use common sense and you'll be fine.

Staying still but being uncomfortable is just as likely (if not more likely) to stop you entering a dream as moving needlessly.

Try your best to reduce movment to an absolute minimum.
If you don't need to move at all FANTASTIC!
If you're starting to ache or hurt, then just get comfortable.

THink about what you are trying to achieve...
Loss of input from your physical body so that your dream body can form and take its place as the focus of your mind.

If your physical body hurts or aches, it will stay the focus.
If your physical body moves, it will become the focus.

Find a balance between the two.
As long as you can make your physical body not be feeding too much information to your mind, then you'll be fine.

hope that helps.