my first build was 100% with a 15w that i paid less than $10 for..
i stepped it up and used a 30w on my next one. For me a clean sharp tip is more important than wattage. The only disadvantage imo is that i had to wait a bit when soldering large joints to let the iron heat up more.
but then again I am ghetto when it comes to building stuff I'm not about fancy tools or using anything that i don't need. some people insist on workbenches and magnifying glasses and wire strippers tinning compound and sterile work gloves. I use my kitchen table, a pair of needle nose for stripping and i usually make sure i have a labatt's within arms reach at all time. I find your joints get cleaner when you have the grateful dead playing, the tone gets more vintage depending on what year you listen to while recording. To nail a 70's jerry sound i suggest listening to the europe 72 concert while soldering. To get a late 60's vibe out of your amp make sure you listen to ladies and gentleman, the grateful dead while handling that iron. Or for the ultimate mojo you can do the 4/29/71 show (live at the fillmore) or the best NFA>GDTRFB>NFA jam ever with the 11-15-71 concert at the austin municipal theater. A nice 25 minute jam to ensure proper time to dress your wires and to double check your layout.
If you want a sterile tone out of your trinity solder whilst listening to an early 80's concert. Make sure you have plenty of days to build on because your work sessions will be short and uninspired similar to jerry's playing at this time.