I have 5 kids and I do own a thermometer, but I don't use it much. Instead I use a book:
"How to Raise a Healthy Child in Spite of Your Doctor" by Dr. Mendlesohn. Here's a link:
http://www.amazon.com/Raise-Healthy-Child-Spite-Doctor/dp...
Dr. M had almost 30 years experience in pediatrics when he wrote this book. It's like our family bible. I can't tell you how many trips to the Dr. and how much medication we've NOT had to spend money or time on- but most important, how much WORRY it's saved me. The amount of worry it's saved me is just priceless!! It is well organized in sections, has a great index- the information on vaccines is out of date, don't rely on it for that, but for everything else I highly recommend this book!!
Read the section on fevers- you will only need a thermometer for after vaccinations (if you do vax.) and if you suspect poisoning or heat stroke (although for either of those you may be going straight to the ER anyway!)
HTH