Percent composition: what do i round a final answer to?
whole number, tenths,

It usually varies for different teachers. mine makes us round to either a whole or a tenth. Also, it depends on the question but honestly if it's just homework I'd suggest using wholes. if it's for something more important you should probably be more precise and use tenths or even hundreds. But, when in doubt it's safer to use as many places as they give you in the question

I suggest rounding numbers to hundredths. Since you're talking about chemistry I'm guessing that significant digits are being used so in that case your number rounding will differ!