Hi KevinM,
As long as the pocket depth is suitable for the thickness of your mattress you should be fine. I would avoid a sheet that is too tight and creates a drum effect but I would also avoid a sheet that would be so loose that the fabric will bunch up underneath you. Most sheets will tell you the thickness of the mattress they would be suitable for and most good retailers that sell sheets will be able to tell you as well.
The fabric in jersey cotton sheets are more stretchy (similar to a t-shirt) but for some people they can sleep warmer and not everyone likes them. There is some good information about sheets in post #7 here and the other posts and sources of information it links to that should be helpful if you are looking for sheets…
A mattress isn’t designed to support the more concentrated weight of sitting … only the more evenly distributed weight of lying down. This is especially true of foam mattresses with softer and more “point elastic” materials such as latex or memory foam. If you spend a lot of time sitting or “propped up” on your mattress then it may be an idea to consider an adjustable bed or a wedge which will help to support your weight and back more evenly along the length of your body when you are sitting up so you don’t sink in as much in one specific area. It would be less stressful for your back as well.
Phoenix