I've been experimenting, recently, with waxed linen thread and our Singer 45K25 and one of our 45Ksv100 corner/box stitchers. Not that they don't work fine with bonded nylon, I just thought that these machines would have been originally designed to use linen, so I was interested to see if there was any difference. I found that, in fact, the stitch does appear tighter and less likely to loosen when you back tack, and, though it may be my imagination, the machines do seem happier with the linen.
However, the pre-waxed thread, I have been using, does require a bit of adjustment on the tension, as the 'stickiness' of the wax tends to increase both the top and bobbin tensions, and the wax may cause other problems later.
Steve