I've been writing checks this way for the last ~15 years or so:
Namely, the amount field is written using numbers like
1,500 and 00/100
rather than the tedious, painful spelling out of
One Thousand Five Hundred and 00/100
99 times out of 100, this works fine. But occasionally you'll get certain rare human tellers who simply refuse to believe this is a valid check and you have to fight over it. I still figure the time I've saved by not writing out all those ridiculous numbers-as-words is still worth it, overall, but it's definitely irritating.
So: who's right? Is a check with the amount written in numbers valid ... or not?