Thanks for the fish.
You are write, one factor that could cause problems, but as long as the temperature over the complete circuit board is nearly constant (and it is) this is not a problem and the currents flowing in the circuit are small. In addition this board is just for my lab experiments and not a (mobile) reference or standard cell replacement.
It could be cased to prevent air flow and therewith temperature gradiants that could causes thermovoltage effects, no question. But on the other hand you need to have the equipment to measure such small effects.
I'm not Jim Williams or some of that voltage nuts