Particularly in the spring as you raise loads of seedlings, you will get far more reliable results if you use some sort of heated propagator. These are low wattage electrically heated trays usually with seed trays on. Often the seed trays have covers to help keep the soil conditions warm and damp - ripe for germination. Some of the Propagators come with thermostats so you can control the temperature exactly as you want it, others tend to just stay warm (i.e. On all the time). You can also use heat mats to just create a generally warm surface in your greenhouse - then you can just put whatever seed trays or pots you are germinating straight on to the heat mat.
Soil warming cables are ideal if you want to make a fixed propagation bed - usually, you would zig-zag the soil warming gable on a bench then cover with sand to create a warm propagation bed. Separate thermostats can be wired in with these.