I'm going to get a new power supply soon, and I've always wondered if 5% more really makes it worth paying an extra $20-30 for.
It depends on if you are paying for your own electricity bills.
Assume 200w average usage, and 5 hours per day. That's 1kw hour per day. At 10c per kwhr 365 days per year, you have a cost of $36.50 per year for electricity.
That's only a couple of bucks savings which will take several years to make up. If your numbers are significantly different, then perhaps it is worth more.
But, a more efficient psu will run cooler and quieter which is worth a bunch to me.