Even Marcus Miller's commented on the dead spot on his famous '77 Jazz. He says that it's part of the character of that bass. He's played it so long that it's part of HIS sound. He just plays that note a little harder (I can't find the Bass Player article this was from, but it was an early BP interview with Miller).
You can't anticipate that it won't be there. How bad it is depends on the individual chunks of wood and metal in any specific bass. So, if it's a big deal to you, don't order from the 'net. Play every Fender Jazz Bass you can get your hands on until you find one that doesn't have dead spot that bothers you. But it could appear as soon as you change strings or do a set-up on your new bass. Or it could go away, or move to another spot.
As explained before, they're caused by resonances that create nodes in specific locations. If the resonance of the neck/body/bridge combination is at the same pitch and out of phase with the string, it's a dead spot. That's why adding mass to the headstock can move the resonant pitch to a note that's higher so it's not as noticeable.
I've played bolt-on neck basses, mostly Fender, since 1976. I managed a guitar store where I was a dealer for Fender, Gibson, Peavey, Yamaha, Guild, Martin, etc. for 11 years. I worked on a lot of basses of all makes in that time. And in my experience I've only come across two or three basses that had dead spots bad enough to cause them to be problems. Yeah, there's less volume or sustain at certain notes sometimes, and sometimes its noticeable. But there are seldom any where it's so bad that it makes the bass worth rejecting. And in fact, two of the bad dead spot basses had other issues that made them less than useful instruments to play anyway.
John
