It is not mandatory. It seems, some of the clergy, priest and bishops, kiss out of their own, individual humility, after the faithful have kissed their right hand. We kiss the priests and bishops hands, because they bless us with that hand. As noted above, the Greek form tends to be, the clergyman kisses the layman's right cheek, after the layman has kissed the right hand of the clergyman, while the layman kisses the clergyman's right cheek; each, then kisses the opposite cheek. A deacon's hand is not kissed, because they do not have the authority to bless with their hand. I would recommend that a layman not initiate the kissing of a clergyman, other than the kissing of his hand, but should reciprocate the kissing of the cheek, if the clergyman initiates it. As also noted above, the Russian/Slavic form, from my experience with OCA and Carpatho-Russian priests who are friends of my Greek parish and either concelebrate or substitute for our Greek pastor, do another kiss upon the first cheek kissed; a third kiss, after the other two have been kissed. Priest and Bishops greet eachother with the exchange of the kiss.