The tub should fit snug, not tight to framing. So if its a compete box your constructing and your placing the tub into this hole your making in the middle you need to measure in from the exterior edge on all sides to be sure your not coming into contact with the sides and deduct that from the tub sizes. The more the top flange is supported the less stress it will have on the entire tub.
When I frame for a tub- if it's a 5' tub, we frame it exactly 5'. As far as your depth, you will probably want to bring your wall out 4+-" further to make your outside corner.
BTW, I have never heard of a 66" tub.
Glkirk, I think she is installing a platform tub. Not a standard against wall type. That is if I understand her question. That may explain the size?