Watch the No Big Blues Bends online guitar lesson by Walter Broes from Rock, Billy, & Boogie Guidebook
This is not a rule as much as it is a general pointer: As with anything, there are big exceptions to the “no big blues bends” idea in rockabilly music. James Burton was a pioneer string bender who started his career playing rockabilly with Dale Hawkins, Bob Luman, and Rick Nelson, and he was bending strings all over the place (albeit with a unique country flavor on top of the blues).
What I'm getting at in this example is that a lot of us fall into the habit of playing typical bends when playing the minor pentatonic scale in a I - IV - V setting, and that it’s worth re-evaluating when you’re tackling rockabilly. There are a lot of very cool riffs and licks involving the minor pentatonic scale where you don't bend any strings, or only use half-step ones. The work of T-Bone Walker and Kenny Burrell is worth mentioning for inspiration outside of rock and roll.