What ever works for you. I have a reason for taking both from the bulb... in a commodore I know the bulb is always grounded and it's switched on by 12+..... but in something else, I don't know. So if I use both.... one of them is going to turn the ####er on. Saves having to test for something I don't really need to know.
In regards to soldering irons... I have 4.
40w for general purpose (I burn through it's tips about every 6 weeks)
60-75 for soldering ignition wires upto 10ga
100w for soldering 8ga upwards
And an adjustable gas iron for mobile work which will do everything, but I won't use it if I don't have too.