It really depends on if I believed that my fifth day would be mine or would the demands of the work bleed into that day? Similar to 20% time at Google - it's easy for your "real" work to take over that time if you don't defend it actively and it's unlikely that you'll be vigilant the whole time.

I think in knowledge work - trading off time for salary is generally not a great move because we're almost always "on the clock."