I back blade all the time at shallow angles with no problems.
When my toothbar is installed I often actually dig material by curling my bucked but do so rather gently without jacking my front off of the ground. Kinda like an excavator, only with the bucket facing the wrong way. Handy to pull down mounds to a loadable position.
IMHO opinion, all manouvers are OK BUT never shock loading (like wham bang), just let hydraulics do the work. They are, after all just like shock absorbers.
For instance, jsut last week I moved rocks bigger than my bucket (never even tried to lift) by simply curling the bucket and rolling them to position and using the bucket with down angle slid them in place to create a retaining wall.
Never lost a one!
Sort of use my bucket heel like palm (to push) and fingers (to hold back) as I slide big rocks into place. Naturally I use teeth to prepare a bed to sit the rock otherwise I risk losing it.
Shored up a large parking area that way an a sloping hillside.
Some rocks I chained down from heights of perhaps 6 ft to manageable positions.
BUT I never 'slam bang' my bucket and always use rather gentle angles and I have logged over 1200 hrs.
Treat the back blade the same way. Gentle cuts, often reverse drag the blade for fine finishing, works like a charm.
Another thing I have done lately:
Back drag my tooth bar and use it like a root 'ripper' to rid the soil of roots and unwanted vegatation prior to loading the bucket.
When I do this my bucket is near vertical and I use float position so that If I snag imbeded rocks or big roots the angle+float lets the bucket ride up and over the obsticle.
IMHO, safer backwards at angle+float rather than forwards as that could shockload the loader arms.