For me:
Since the backing line is there simply to make the spool's arbor diameter larger, and (in most cases) you've spooled on a sufficient length of new filler line, and will likely never have enough line out to reach the connecting knot, or will replace the filler line when it gets low (from breakages) or starts to deteriorate from age ..... most any knot can be used. This "backing to main line knot" shouldn't get exposed, even after letting out the greatest amount of main line that you'd ever use.

Generally, I pull the two tag ends together, side by side & make a loop, then run the tags thru the loop twice, then wet & cinch ... then trim the tag ends as close as possible to the knot. I've never been "spooled", nor have I ever let out enough line to reach the knot .... so my concerns have always been about the knot(s) that I tie my baits/setups onto the main line with. A lot of people are concerned about the possibility of being "spooled" (having a large fish pull off enough line to reach the backing or spool) ... but, I'd be more concerned with the condition of the backing line, than the connecting knot, in that scenario. Unlike the spools of line that you have at home, stored in a cool/dark/dry place, the backing line has been in the elements ... subject to heat & cold, getting wet & drying out, and possibly even getting some UV damage. That's why I totally replace the entire reel spool, about every two to three years, with either totally new line or 1/2 with seldom used line & topped off with new line.

... cp