My daughters reckon I'm turning into an "anorak", but I've really got the thermometer habit. You can buy maximum/minimum thermometers for around £6 - I have 2 (1 inside and 1 outside) and I actually keep a record of the daily temps. Most seed packets give a recommended germination temperature, and even without a thermometer I find that a kitchen windowsill does the job for most seeds. I only use my little heated propagator for things like tomatoes and chillies (and courgettes)