Copy/ Paste does work, but you shouldn't simply copy/ paste the entire Oracle home. This is a lot more effort than what is required.
You will firstly need to perform a log switch, i.e.
Connect / as sysdba
Alter system switch logfile;
Place all your tablespaces into backup mode, i.e.
CONNECT / AS SYSDBA
ALTER TABLESPACE mytablespace BEGIN BACKUP;
(You can get your tablespaces by querying the DBA_TABLESPACES view)
Then copy all your data files and redo log files to your backup location.
In regards to whether this method is safe or not, It depends on how you are retaining the data files and log files. Of course, I should mention that RMAN is Oracle's proven and recommended mode of backup.