Good question. Is the cupboard the same size as the fridge? Is there a heat source, other than the fan motor, in the cupboard? Is the cupboard well insulated and draught proof? How small is small temperature increase?
Sorry for all the questions but there are a lot of variables that need to be considered (or not).
I've never done a detailed evaluation of the temperature of the wort in my fridge. My opinion is that convection inside my fridge will circulate hot and cold air. The thermal mass of 5 gallons of wort is large compared to the thermal mass of the fridge atmosphere, so fluctuations in air temperature will occur faster than the wort can react and will always lag behind. So while the air temperature may vary by plus or minus 1C, say, the wort temperature variation will be much less. My opinion. No data to prove it.
