If you want to roll forward an Oracle DB from a snapshot using future archives, then you need to put the database in backup mode before taking a snapshot. If you want to use the Oracle DB from a snapshot as is, then you don't need to to put the database in backup mode.
Sample PL/SQL script to put the DB in backup mode before taking a snapshot:
su - oracle << EOF
sqlplus "/as sysdba" <<END
whenever sqlerror exit1
alter system archive log current;
alter database begin backup;
Sample PL/SQL script to take the DB out of backup mode after taking a snapshot: