To get the best shape on a dart you will not want to sew it on a straight line, even if the pattern suggests so. This is to avoid a sharp corner at the tip. You do not want any pointy bumps in your bust area! See how I’m not following the drawn white line towards the tip but giving it a nice round shape instead.
I back tack my dart tips although the general rule is to tie off by hand. If you’re experienced enough and the fabric is not too thin I think back tacking is acceptable. Don’t overdo it though, a few stitches is enough.
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