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I know this is a very basic question, but I am curious if you fellow seed growers are leaving your heat mats on 24/7 or you have an overnight cooling period? I know many species have different needs but is there a general thought on this matter?

I put mine on a 12 hours on/off cycle. I haven't really experimented, but it seems fine for germination and good growth.

I have mine on 24hrs. So far everything looks good and happy.

No firm answer to this. It depends on temps in your rooms and heat preferences of the grown seedlings. If lamps produce heat and that is enough to create day/night fluctuations you can keep mats on but if seeds need low temps to germinate better switch it off night time. Most times I was doing seedlings the mat was on in winter months and fully off during the summer. Monitor your compound temperature.

I always use a cheap low-power mat that just raises the temperature about 10*f, and leave it on all the time. No need for a thermostat with that, it's not going to get too warm - if the weather gets very hot I unplug it. It's not in a climate-controlled space, the temperature fluctuates naturally. I start seeds only in the summer, obviously, the extra heat is needed only because our Pacific Northwest climate doesn't reliably provide enough warmth.

I leave mine on 24 hrs/day. I've wondered if I should turn it off at night. I don't know if it makes a difference. I think on/off cycle if the lights the lights can provide some temperature variation. My temperature sensor is under/between the pots, so it keeps the temperature constant there, allowing (I think) for some temperature variation at the top from the lights

Ideally after germination the heat from the lamp slightly dries the surface during the day, and moisture wicks back up again overnight and condenses and runs down the bag to the surface. Leaving bottom heat on kind of counteracts that during the day. If anything I'd supply bottom heat at night and lights only during the day. Getting that daily moisture movement just right makes growing slow stuff like strombocactus very easy, suppresses algae, moss and mold, and perfectly matches the conditions for baby cacti in habitat.