First off, if your are set on a parity raid I would recommend a RAID 5 with an automatic backup scheme over a RAID 6. The backup could be over multiple drives (recommended), a RAID 0 (not recommended), or another RAID 5. This carries a good balance of cost and safety for your data because it can also handle controller failure and human error.
What you will be safe doing depends on your array size(s)
Many in the IT industry are moving away from RAID 5 for two reasons. First, the shear size of many of the array's require extremely long rebuild times which opens the window to long for a second failure. Second is the issue of a URE which is an unrecoverable read error that is not known until a block fails to be read and then the entire array on that drive becomes unreadable. Again, UREs happen rarely in terms of shear number of errors per bits read, but when you have TB size arrays, rarely is too often. An array cannot be rebuilt from a drive with a URE. So if you have a failure, and a URE, you effectively have 2 failed drives. RAID 6 helps with this because you can lose two drives and still rebuild the array.
But, if you have a 100GB array in a RAID 5, these problems diminish greatly and a RAID 5 becomes more viable; however if you have a 100TB array (just as an example) on a raid 6, you might need more redundancy or another option!
RAID 5 with TB arrays is definitely a gamble and I would not trust it. And it does not make sense to run a RAID 6 with 4 drives over a RAID 10 unless you are going to be expanding your array soon and need to do so 1 drive at a time for cost. And let's not forget that you can run multiple RAID 1 arrays, it just segregates your data into separate partitions/disks you have to work with and takes away the convenience of one big drive.
But everything has its caveat. The larger the RAID 5, the higher (exponentially higher) chance of a rebuild failure due to second failed drive or URE. RAID 6 higher cost and complexity. RAID 10 higher cost yet (but for simplicity). And multiple RAID 1's cost of convenience.
In the end, you need to identify your needs, segregate what you cannot lose, and what you are ok losing. Match that with a budget and your level of expertise while choosing which array to choose.
Us, we put our office and home data on a RAID 1 (2 drive RAID 10 with MD RAID actually) for safety with daily backups. its simple, fast, and reliable. Any other data like video files or other large non critical files we put on a RAID 5 or single drives with occasional backups based on how important the data is because we do not want to afford the same level of safety with our movies as we do our family photos and/or our companies' files.