ITA about having no kids to try stuff on.... My boss is having a baby, so I picked the 12-month size, and am almost done. I'm eyeing the sleeves, going, "Is that long enough? Is that too long?" I figure better too long than too short, and if I don't make gauge perfectly, well, it'll fit at SOME point, because the baby isn't even here yet.
You can look at it this way - you're not trying to knit for a special occasion, right? It's not something that has to fit for Christmas, for example. As long as it's bigger than the baby and versatile enough to be worn in summer or winter, it will work. So use the yarn that makes gauge and you're comfortable with.