My method is just a variation of the first suggestion. I insert the sockets, place the board face down, and then very lightly "tack solder" pins 1, 5 and 9, just enough to hold them without permanently commiting them to a fixed positioning. Sometimes it's easiest to do the EL84 sockets first, then the two inner sockets, so you don't have to juggle six sockets, which wil ALWAYS result in a lot of frustation and utteing some choice "colorful metaphors" in the process
If you have already punched out the holes for the tube sockets, temporarily mount the board into the chassis, then look at the top. If any of the sockets need re-centering, they can easily be shifted around to "dead center", once you have accomplished this, then fully solder all of the pins.
I'll "second" the idea using some "plinker" junk 9 pinners in the sockets to prevent the solder from flowing into the contacts (been there, done that)...
/ed B