It is highly unlikely that the piglets would drink replacement milk
1. they're getting all they need from mum
2. There stomachs are tiny at this stage, they take only a few ml each time they feed from mum, which is why they feed so often.
3. If they did drink, they would be getting second best, and take less from mum - remember "breast is best"
4. they only drink when the "milks coming" grunt comes from mum, and they need to suckle not drink, so will ignore it
5. You more likely to introduce infection in milk if you leave it lying around.
So yes stop fussing