Actually, the reason people sleep on their back is because sleeping on your back can increase the chances of sleep paralysis, and the latter is the preferred way of achieving AP by many people. In my experience, getting out is the same in difficulty regardless of position.
To Blakey: Again, it depends on which method you are using to achieve astral projection. In some cases, it does not matter at all how you sleep. For example, if you wanted to astral project via lucid dreaming.
Based on your question though, I'm going to assume that sleep paralysis is the way you're trying to go. In this case, it's first important to know what sleep paralysis is. It's related to a safety function that your body executes every night, called REM atonia, so that you don't put yourself in danger by acting out your dreams while sleeping. With that being said, it should be cleared up that there is no "best" way to sleep. Your body automatically does this every single night. Sleep paralysis is when you wake up while this is happening. The goal is not to sleep a certain way. The goal is to do something that will keep your mind awake when your body goes into REM sleep and paralyzes itself.
Not necessarily, you can do the rope from your side as well. You can even imagine a rope back from your head-while being on your side, and think you reach your hands and you pull yourself out. You will seperate from your head and upwards... Be creative.
Sleep paralysis mostly occurs when you break your biological clock of sleep-if you change the hours and program of your sleep. You can't cause sleep paralysis like you induce trance, because SP occurs by that "breaking of bilogical sleep clock"... You may search on internet for that, it'll make you understand. To induce SP you either have to do that, or do it through a different way like drugs. Additionally, SP causes a lot of hallicunations... You may have the craziest hallicunations from many sides, such as fearful or weird... If you see all these it may be much harder to focus and seperate properly.
If your back and neck have an ache then you won't be able to focus appropriately on the whole process, but you may place some pillows in the appropriate possition in order not to have this isue.