Ventilation is very important in your greenhouse or polytunnel. You will find that a polytunnel will probably warm up sightly quicker in the sun, and normally does not come with ventilation windows, unless you build some into the frame, which I will be doing with mine sometime in the future.
The idea is to keep your greenhouse from 20 to 25 degrees maximum, although my polytunnel does get a lot hotter even with both doors open. I mist the polytunnel on very hot days to keep the temperature down a little though.
So the obvious choice would be automatic ventilation which will take care of itself. However, if you have not got it and you are out at work all day, then you must decide in the morning what action you are going to take.
Roof ventilation is probably the best, as heat rises and you will not get wind damage, but if you have not got it, on very windy days just vent the less wind side, or the end.
Thermometers, yes you definitely need one. But make sure that you have one with maximum and minimum temperatures on it. They are not that expensive, and you will know what has happened while you were not around.
Cold days, easy option here, keep everything tightly shut.
Rainy days are fine, you could just leave a crack of air so there is not too much condensation. However, if you feel it is going to brighten up then leave a door open, just in case.
Windy warm days, you have to be careful not to get wind damage on your plants, but it will also get really hot so you will still need ventilation. My soloution is to have fine netting on the back door of my polytunnel on the outside of the frame, this cuts the wind, but still cools the tunnel. On the main door, you can build a second door with netting in it.
Very hot sunny days, you might not be able to cool the tunnel or greenhouse sufficiently, but do not over worry, as when I worked in Portugal it got extremely hot in our large polytunnels, with no real detrimental effect. However, as I mentioned before, we did mist the polytunnel quite a few times a day, this will keep humidity up as well, and as long as the plants are dry by night, you should not have any problems with mildew.
I feel it is better for the plants to get slightly cold rather than too hot, so in the early spring when the sun can suddenly catch us out, and as long as it is not too cold, then I would open the doors in the morning before you go to work. In the height of summer, I tend to leave both doors open day and night.
The gate is to keep out the dog and the horses!