I didn't put anything as my 5th and 6th choice. I knew I wouldn't settle for anything other than medicine and would have taken a gap year if I didn't get in, so it was pointless and a waste of the Universities time!
Also, although they say it will not make your application any less favourable; and universities cannot see where else you've applied. If you applied for say biochemistry and medicine at Manchester... the medicine people at Manchester would be able to know that; and I feel it may (even if only subconsciously) mean that it may hinder your applicant slightly: as you would be willing to take an offer that isn't medicine.
Saying that though: I know many people that applied for other healthcare subjects in their last 2 options, being at the same universities they applied to do medicine: and still got medical offers from them universities!
Basically, if you would be willing to do another agree other than medicine put one down: but, if you're never ever going to do a different degree it's wasting your time and the universities in my opinion.
[Oh, and some options to apply for if you did want to do some choices are pharmacy, pharmacology, biochemistry, microbiology, neuroscience, physiotherapy (at a push), physiology, optometry, audiology]