I recently acquired a seam allowance guide, which magnetically attaches to your scissors and serves as a reference point for how far away the scissor blades are from the pattern pieces, ideally eliminating the need to mark seam allowances on patterns that don't include them.  The key word is "ideally".  After a short trial, I decided that my own visual estimation of a 5/8", 3/8", 7/8", etc. seam allowance was more accurate and more quickly cut.  The protruding magnetic guide seemed to be getting in my way more than anything.

I'm disappointed that I spent money on a tool that I won't get much use out of, but its role in my life did one amazing thing: it gave me the confidence to trust my skills and eyeball it!

I feel like I've graduated (from the sewing version of about Kindergarten), or taken one step up the ladder of mastery of my craft.  It reminds me of an article I read years ago about a furniture maker, who explained that through repetition, he'd developed the ability to measure with his eyes, and that rulers actually hindered his craft and he didn't trust them as much as his eyes!  I found that amazing and yet somewhat unbelievable....until now! :)