whenever you sit and idle, the radiator doesn't get air flowing past it like when you're cruising, so it is less efficient at removing heat, so yea the temp goes up. how high up it goes is up to the efficiency of your cooling system. i've hit 215 idling in the summer for 5 minutes at a light. no biggy, aluminum needs much higher than that to warp
if you're staying at 145, check the fans too. when the coolant temperature sensor is fucked, it sends screwy signals to the pcm, and commands the fans to be turned on. your temperature guage is not based on coolant temp, hence why you can pull the plug on the ect and still have a temperature reading. 145 sounds like the fans are on AND the thermostat is flowing too soon. even if the therm is open too soon, or perhaps stuck open, check something: how quickly is your car getting to 145? slow, like 10-15 minutes of driving = stuck open, fast, like normal driving of 5 minutes or so = working, but out of its specifications, aka opening too soon.
if you're fans are on when they shouldn't be, it's usually the temp sensor. if it is operating within its range, aka working but is slightly off, it'll command the fans on too soon (cool). fans for the cobalt SC turn on at 225 degrees (tested @ my school's auto lab)
get your car up to 145-160 where u say it stays. pull over, and see if the fans are on. if they are, check the voltage @ the etc. you can carefully backpobe it, and tell me the voltages you get since i tested mine. replace if necessary. the fans shouldn't come on too soon now, and your temps will return to normal. if they don't, change the thermostat too. then you'll have peace of mind of a properly flowing thermostat and a properly working coolant temp sensor.
as for the 'tvs reservior' what are you referring to? did you drain the fluid?? when the thermostat opens and routes some more fluid throughout the system, you'll need to top it off after it cools. 2 quarts going to nowhere's land is fine... unless you keep adding and adding, then you have a leak or a blown head gasket, which i doubt.