The 5 to 6 percent figure is not scientific, it is based on several members sharing their log file statistics in a Cre8asite Forums thread.
Those that did the leg work to figure out how many people they see come to them via a "did you mean" from Google is on average between five and six-percent. Typically, people are noticing a lower percentage, between one and two-percent, using spell checking features on Yahoo.
Here are some quotes from the thread:
I noticed that an average of 5 to 6% of all visitors from Google used the spell-correction to make it to my site.
Google: 6.7% misspelled, 0.4% I'm feeling lucky
Yahoo: 5.6% From related (rs=0), 1.0% Misspelled (SpellState)
The related does not mean spelled wrong but the misspelled does, in Yahoo's case.
Misspellings in Googe: 5.3%
Misspellings in Yahoo: 1.7%
Misspelled Google: 4.3%
Misspelled Yahoo: 0.8%
It is fairly interesting look at search referrers. Maybe one day, I'll dig deeper and take a look in our statistics for these metrics.
Forum discussion at Cre8asite Forums.