How are small interest payments paid back?

hi,

does anybody know how small interest payments are paid back?
lets say you invest euro 10 for a long time
and per month you get 0,00489  
is that rounded down to 0, so you get nothing?
or carried over to next month?
etc?

If you only look at rounding the amounts paid back, does it matter if you invest ten times euro 10 (for statistacal purpose) compared to one time euro 100?

Thanks,