At my school there were two university advisors with very different attitudes to this. The first let people apply wherever they liked. We had three unspectacular candidates going for Christ's, Trinity and Emma, probably the toughest Cambridge colleges. Not one got in, but all of them got pool offers and they're all now at Cambridge.
The other was constantly stressing people about difficult colleges and even difficult courses. It got to the point where one year no-one applied for a certain fairly popular course because he'd scared everyone away! And virtually all the unspectacular candidates who applied to supposedly 'easy' colleges still didn't get in.