I don't believe it would hurt your resume, but it would definitely help. Speaking from experience, becoming a CFI/CFII has made me a better pilot IMHO. Anything you can add to your resume, and 'pad' it so to speak, will make you stand out from others that are applying for the same competitive job(s). I understand that instructing isn't for everyone, as I've seen many CFIs that don't teach their students much, and just milk their students to build their hours. With that said, if you feel like you won't be a good instructor, then don't do your students a disservice by being a crappy instructor. Get your instructor rating(s) don't instruct if you don't want to, but don't let them expire until you get to the airlines,