If the maximum current value of an ID generated by a sequence is N, does that guarantee that all future rows will have index > N?
If the database is running on a RAC cluster, each node in the cluster would have a separate sequence cache.  That would cause the values returned to be out of order unless the sequence was created with ORDER specified (i.e. CREATE SEQUENCE seq_foo START WITH 1 ORDER).
If the database is running on a standalone instance, sequence numbers will always be generated in order.  If you're depending on this behavior for functional reasons, I'd be pedantic and specify ORDER when creating the sequence even on a standalone database.  But it isn't necessary to do so.
From the documentation

ORDER is necessary only to guarantee ordered generation if you are
using Oracle Real Application Clusters. If you are using exclusive
mode, then sequence numbers are always generated in order.

The other potential issue would be if the sequence reached the MAXVALUE and was set to CYCLE by starting over at the minimum value.  NOMAXVALUE and NOCYCLE are the defaults, though, so it would be pretty unusual that someone using a sequence like this would create the sequence in that way.