I have never undertsood this attitude. I get this at work. "nah mate, I'm not going to do any overtime this week as it will put me into the top tax band and those extra hours at double time I will get taxed another 8% so it isnt worth it"
Because it devalues the per-unit of your labour. Imagine you got paid £20 per hour. Then on Fridays your boss told you that they were only going to pay you £10 per hour. You would be less inclined to work as hard or as long on Friday than you had been for the week up until that point.
The above is a correct analogy - bumping up into the next tax bracket has exactly that devaluing effect on your labour even though the total value still increases somewhat.