You should definitely not rely on never missing a value in a sequence, as they optimise for concurrency over sequential numbering. There are quite a few situations in which a number can be "lost".
Furthermore, the value visible in the dba_sequences may not be the actual next value, as the numbers are assigned from an in-memory cache. The underlying sequence metadata table has no data on the usage of that cache. You should also bear in mind that in a RAC system each instance has its own cache of sequence numbers.
You might describe the problem you are trying to solve, as it could be that sequences are not an appropriate mechanism for you.