Generating consecutive sequence values

I'm using a sequence to generate unique 8 digit record keys which will be
given out as membership ID's.

Because we want to be able to pre-assign blocks of membership numbers
for use in off-line membership registration situations, I need to be able
to ensure that a block of sequences are in consecutive order even though
they may be requested concurrently with other activity on the sequence.

If I use SQL such as "select nextval('id'),nextval('id'),...,nextval('id')"
to allocate 50 or 100 sequence numbers, will the backend ensure that
those sequence numbers are a consecutive block?
--
Mike Nolan
nolan(at)tssi(dot)com