If your words aren't completely clear, the only reasonable thing the bank can do is ask you to write the check again. And as has been pointed out, the example given is less unambiguous than the OP assumed.
For what it's worth, back around 1970 I was explicitly trained to write the number in words from the very start of the line, preferably WithCapitalizedInitials rather than with spaces between words, to draw a line through any unused space, and to always use a specific kind of phrasing, all to prevent both misunderstanding and forgery-by-alteration. For example,
SixThousandFiveHundredThreeDollarsAnd21/xx---(Dollars)
The word Dollars is optional since the check labels that line as Dollars, though I often write it anyway.
Zero cents could be indicated as No/xx, 00/xx, Exactly, or more commonly by starting to strike through the rest of the line immediately after the last word so there was no space to insert pennies. (There was a time when pennies were much more significant...) I lean heavily toward Exactly.
The only part of that which is at all informal is the convention of using /xx as standardized alternate notation for /100.
There is nothing wrong with informality, but legal and financial documents are generally not the right place for it. Yeah, it would be nice if we could be sloppier about it, but the rules are there and get enforced for good reasons. If you want to be less formal, there are always credit cards; they don't use the numerals-versus-words crosscheck at all so the issue doesn't arise.
(By the way, if anyone else's script has degenerated as badly as mine has: while I was taught to write the check in script so a break in the continuous line could help expose an alteration, I've been using "printed" handwriting on checks for decades, for readability's sake.)
34.95
, but someone else extended the9
to subsume the decimal point and add a.00
? (I've always written the cents, even when.00
, as a superscript over anxx
to make it clear what the cents are.)