I start with a 6 pile shuffle and do a thorough overhand shuffle afterwards. In case of a mulligan I distribute the cards into the stack and do another overhand shuffle.
 
 
 
It is. Couldn't find the exact section it says so about the initial shuffling, but the Mulligan explanation in the tournament rules (page 2) states:
 
Mulligan Rule
 
During setup, a player may re-shuffle his opening hand into his deck, pass the deck back to his opponent for additional shuffling or cutting, and draw a new starting hand from the same deck.