Okay, so maybe Jeff needed a little more tact in his original post, I'll second that opinion. Nonetheless, he typically does give good advice and this is no exception. When I read your original followup (before his response), in my own head, I was drafting a response and it was going to start out: T-H-E-R-M-O-S-T-A-T...so Jeff and I had the same thinking on this one. Thermostats are KEY in maintaining a comfortable and accurate environment for your herps. Sorry if that is insulting you, I totally don't mean for it to come across that way (and anything I say here for that matter...if I cover information that you are well aware of, then I apologize but maybe there are people out there that aren't aware and this will help them).
Unless your indoor temps get so hot that it's actually hotter than your hotspot temperatures (let's say you have your ball pythons and you want them at 89 degrees but your inside temp is 92 degrees), then a thermostat will help. In a case where you're running 60 degrees in the winter and 95 degrees in the summer, then I'd suggest you climate control at least one room in your house to be suitable for reps, or find an alternative spot to keep them. Unlike running 'open' or using a rheostat, a thermostat will read the heat and act accordingly. Running wide open without a rheostat or thermostat will definitely be a problem, not only are a lot of heating methods too hot for a reptile when running full blast, but it will also be susceptible to outdoor temperatures as you mentioned. A rheostat is one step better in that it controls the amount of voltage (if I'm correct there...I apologize if it's th wrong term, I suppose it could be amperage) going to the heating element. However, you set it at say 80% and it will maintain 80%, it has no 'brains' to know when the inside temp has gone from 76 degrees to 84 degrees. The thermostat is the ideal methodology to use. It actually monitors the heat source with a problem and maintains the temperature at that level.
Within the thermostat world, there are two types, proportional and non-proportional. A non-proportional thermostat (A-Life, ESU, Ranco, a ton of others) work with an on/off mechanism. Say you have it set for 89 degrees, when it reaches 89 degrees, it shuts off, then it goes down to a temperature (typically one or two degrees depending on the thermostat...Ranco's are actually programmable) and then kicks back on. So you're hotspot is actually between say 87-89 degrees. Those thermostats run from $25-$100 dollars (Ranco running on the top end of the scale there). Then there are proportional (or pulse proportional thermostat) thermostats. This would include Helix, GCS, and Big Apple's Herp Power units. Those thermostat vary the amount of voltage (again, I could be wrong on the terminology) going to the heat source so that they maintain almost a constant temperature. They are effectively on all the time, but sending voltage pulses less frequently when up to temperature and more frequently when below temperature. I believe the 3 thermostats I noted above have less than a 1 degree variance in them and they are truly amazing (and highly suggested for high dollar animals and for incubation purposes). However, they do carry a price tag. Those thermostats run from $110-$150 depending on the model, night-drop options, etc. I'm personally a big fan of the GCS 4-zone thermostats. I do like Helix units, but dollar-for-dollar, they come out way ahead. 4 independent proportional zones with built-in night drop AND an alarm system for $259! You can't beat it.
Anyway, sorry again if this is stuff you're well aware of. In your response back to Jeff, you didn't say whether or not you actually did have a thermostat. If you don't, I'd highly suggest it. 95% of the fluctuation that you see will go away (at least on the hot spot, obviously the ambiant air temps will vary depending on how you heat/cool your house).
Take care,
Jason @ Jason's Jungle