Petticoats though hardly a new phenomenon on the fashion scene are showing up more and more in bridal wear as both understructure and showpieces--that is, peeking out from underneath skirts sometimes sporting different colors and textures. A little or a lot of crin peek-a-booing beyond the hemline is sexy and has the element of the unexpected once you make your entrance. Above is a taffeta Aline flounced below in ivory and black. Below: An organza lace dress is accented with a Chantilly lace under slip . . . .