I managed to install my new baseboard this weekend. Just under 24' worth, and with the associated piping down cellar, over 50 sweated joints. Filled it up yesterday morning, and after breaking off an air valve on one of the radiators upstairs, drilling, tapping and a trip to the hardware store, there was not one single leak.
I really took my time cleaning and preping each joint. I brushed all the fittings and emery-clothed each pipe as i was fitting it up, and redid it before i re-fit and fluxed everything.
Thanks for the advice and tips, everyone!