FWIW, with slice-of-life anime like K-On, there's likely to be much less cognitive dissonance than, say, seeing Bart Simpson's voice coming out of Nancy Cartwright's body. Even with the age difference between seiyuu and characters, the costumes and hairstyles and such go a long way towards putting them in-character. (It also helps that seiyuu for shows like these are often the same ones doing the vocal performances for the opening and ending themes and, in this case, the songs the band performs.)

YMMV, of course.

>Mark Hamill's Joker is a good example.

<tangent>

Mark Hamill's Joker voice has been a source of cognitive dissonance for me. He used the same voice for his appearance as the Trickster on the John Wesley Shipp-starring live-action Flash series from the early 90s, and I've also seen him do the Joker voice in interviews and other live-action bits (such as this one). But I cannot reconcile him with that voice. In that linked bit, although my brain is perfectly capable of assigning Harley Quinn's voice to Tara Strong (along with all of the other voices she's done), my brain is adamant that Hamill must be lip-syncing or something, because it just refuses to accept that it's coming from Luke Skywalker.