I still have an unorthodox grip. It drove my mom crazy for YEARS. (Still does, actually, if she has to watch me write.
)
It didn't hamper me at all. I was a lit major (including years of graduate-level work) back in the days when we still wrote by hand, and I took better, faster notes than anyone.
That's purely anecdotal and indicative of nothing about your child, of course. But to answer your question, yes, I personally went through it and came out fine on the other side. I do have pretty ugly handwriting though...
Oh, and now that I'm teaching my son cursive, I find that it fatigues my hand quite quickly. That's certainly a point in favor of proper form. My usual writing looks more like italics, or a weird hybrid of cursive and printing, and that style of writing I can do for hours with no ill effects.
