Yes, the tubes will burn out early (if too low) or melt
their screens and short out (if too high) But they'd
probably do that anyway! There was a guy posting here
a while back who put 6V6s in his 6L6 amp, and they
melted the minute he cranked it. 
 
So to help your tubes last longer than 5 minutes  it
would be wise to replace the screen resistors for bigger
ones, like 2.2k. That's what this guy did, and I think it
worked.