The currency format is very restrictive and needs the certain number of decimal places for a given currency. As you can see here there can be different number of decimal places depending on currency. For most currencies it needs two decimal places though. But, you can trick the parser. Just try it with standard number of decimal places and if it fails try it with one decimal place again. Your code would look like this then:
var amount = dojo.currency.parse(stAmount,{currency:"USD"});
if (!amount && amount!==0) {
amount = dojo.currency.parse(stAmount,{currency:"USD",places:"1"});
}
This accepts inputs like
12
12.3
12.34
$12
$12.3
$12.34
But it is still very picky. It doesn't accept spaces or more decimal places then currency allows.
If you want more flexibility for your users and need USD currency only I'd go for dojo.number
or other number parsing instead and show the "$" outside the input field. Then, you'd be able to accept much more formats and could add functions like rounding.