You know when you use a heatblock with no lid and all your sample ends condensing in the lid of the tube? That's why they invented the heated lid. It will discourage your sample to evaporate and condensate on the lid, where the temperature is lower than what you wanted it to be. In the old days of PCR, when heated lids were not available yet, every 95oC cycle would cause evaporation and loss of sample, so after 25 cycles, you had nothing on the bottom of the tube. So they overlaid a droplet of mineral oil on the reaction, so that it wouldn't evaporate, then they had to extract the reaction without the oil, that was probably a big pain! Then a very smart person invented the heated lid, fixing everything.Use the heated lid, it won't hurt your reaction. It doesn't even need to be a high temperature, just high enough to discourage condensing.