I've got a 10g tank and I purchased a python. It really is worth it if you can afford to drop the money on it. It saves so much time and a ton of headaches.
I add dechlor while I'm doing the fill, I figure that way that it will get mixed around well and do its job. I don't think it really matters when you do it though. Like David said, I try to match the temperature as best I can before switching it to "fill mode". I don't use a thermometer, so I'm not very accurate with it.
@theotheragentm
99.9% of sinks will work with a python. The tip of the facet will almost always unscrew (if it isn't one of those new age fancy pants sinks). Just get an old t-shirt/rag or something to keep you from scratching up your fixture, put it around the tip of the sink, grab ahold with a pair of pliers, and turn it to the left (righty tighty lefty loosey). They usually come off pretty easily.