It depends on a lot of things:
Type of db: OLTP vs. OLAP
Size of db: 2GB vs. 1TB
"High usage databases" I am assuming means an OLTP type of database not an OLAP db. With an OLTP db your data will be changing rapidly and I would recommend doing hourly transaction log backups with a daily full backup (if possible, depending on the size of the db).
If the size of the database is relatively small, (like 2 - 10 GB) doing a full backup once every 6 hours is okay. But if the database is larger in size I do not recommend doing full backups during transaction hours.
Realize that if you decide to go with backup every 6 hours, you run the risk of losing up to 6 hrs worth of work if something bad happens, as to hourly tlog backups you will only lose 1hrs worth of work. There is a price to pay however for hourly tlog backups, and that is more maintenance, and it will take longer to do a full recovery and then apply several tlogs.
Consider both the business and technical side of backups