In reply to Frank the Husky:
It's generally considered OK to place pegs on new routes and replace them if necessary on old ones on the North Coast (Atlantic Coast and Culm), however, since the things rust, it's better to manage without them if you can. If you place them, it's worth considering if it is better to take them out after the climb rather than leave them in-situ. This is debatable - is the route likely to be repeated before the pegs rust away? If the peg rusts in situ will it still be possible to replace it?
It is not, however, acceptable to place pegs on existing routes where there have not been pegs in the past. (A lot of unnecessary pegs seem to appear from time to time on Wreckers' Slab, these often get "harvested" by local activists for use on their own routes.)
Note, none of the above applies to West Penwith - this was debated at a BMC area meeting some time ago, at which it was agreed that no new fixed gear should be placed and that any existing gear should be left to rust in peace.