Is there a format for numeral.js that will show decimals only when needed?

Yes. Put the optional decimals in brackets:

numeral(1234).format('0,0.[00]');
// output: 1,234

numeral(1234.567).format('0,0.[00]');
// output: 1,234.57

Either no decimals or 2 decimals:

numeral(1234).format('0,0[.]00');
// output: 1,234

numeral(1234.5).format('0,0[.]00');
// output: 1,234.50

(Based on johnwp's comment from the accepted answer).