Is bottom heat equally important when germinating flower seeds, or does my usual method of placing the seed trays under grow lights in a 70-degree room produce the same result?
Different plants like different temperatures to germinate and soil temperature can influence the speed and sometimes the amount of seeds that germinate.
80-85 degrees F is a good temperature range for many seeds fast but most will germinate at lower temperatures albeit a little slower. For some flowering plants that I'm aware of 70-75 degF is the ideal range.
My best advice would be to search for something like "Germination temperature [plant name]" and you'll likely come up with some links to academic research or instructions from seed producers.
A propogation mat isn't necessary but is nice to have, especially if you have a thermostat controller to go with it. The controllers are a bit pricey. If you don't mind fiddling a bit, a soil thermometer and one of those 24 hour plug in lamp timers that allow you to set on/off for each 30min increment could give you adequate temperature control. The lights will also generate a little heat as well. You might want to set up a you table and lights and put a thermometer under the lights and see what the day/night temperatures are under the lights. You may find you don't need a lamp.
In the past I used to just set up a small table near a radiator and it provided a nice warm environment. You have to be careful to place the table in the right spot so it's not too warm or you'll bake the seeds.
This year I got a seedling mat to keep the soil warm. In the past my vegetable seeds would germinate in as little as 3 days. Same with the mat this year.