nah, theres no specific structure... as with any music, its up to the individual. alot of tapping solos for rock is based on the pentatonic scale. as with most solos. think of tapping as just a way of making a normal solo more exciting.

you can also think of tapping as a way of making legados with wider spacings inbetween the notes. sometimes a tapping lick can span an entire octave, while a typical legato would probably do about half of it.

go here: http://www.guitarshredshow.com/ and go to the beginning of the tapping dwarfs thing... and eventually you should see something that says "print tablature" on the right. print all of em out. theyve got some sweet tapping licks in there. you should see the pentatonic theme going.

just try that out.