I've been knitting for over half a century (I'll be 60 next month and I can't remember not knowing how to knit) and I've got some really firmly ingrained habits. Two have really made themselves felt in the last year. These are casting on and off loosely.
I reflexively begin everything I knit with a loose two-strand cast-on, unless the instructions say otherwise. For big yarn I put the ball of my right thumb between the new stitch and the previous stitches to make a space as I pull the yarn in my left hand up and for smaller yarn I just don't let the left-hand yarn slide too close to the other stitches. It's a nice flexible cast-on anyway and I've never had a problem with tight edges.
As for casting off, I work those stitches very loosely. I'll even consider using a larger needle and I never tug on the yarn between stitches to tighten them up, just a bit to keep them even. I've worked other cast-offs, too, and I'm reasonably proud of my nice flexible, loose edges. No one will ever pull their ears off putting on or taking off a sweater I've made....
Well, these habits have caught up with me. They produce flared hems on my scarves. I'd like to blame the non-elastic yarns I've been using for scarves, but I had the same problem with wool as with the others. Maybe it's the difference between the ribbing and other edge treatments that garments use and scarves don't, but I just don't know. I've tightened up my casting-on just a bit and I've really started snugging down my casting off. These would be crummy hems for actual garments, but they work pretty well on scarves.
However I'm now a little worried about what will happen when I knit something that isn't a scarf and that needs a relaxed hem. It took me a while to develop my technique and now I'm undermining it constantly. I mean, I like knitting scarves but there's going to come a day when I need a relaxed hem. I hope it's like riding a bicycle, that you never forget.