1) Music is the glue that holds a ceremony together. Many couples understandably want a great variety of music during their ceremony, but don't forget that processionals take mere seconds (15-30 seconds max), so choosing a separate song for Seating of the Grandmothers, Seating of the Mothers, Bridal Party Processional, Flower Girls, Bridal Processional (you get the idea), will allow too little time for your song choices to be heard. Using a different song for each portion may greatly impede the flow of the ceremony. Stick with 3 songs at most: Seating of Grandmothers/Mothers, Bridal Party, and Bridal Processional, and fit everything else into these.
2) The Music should fit into the context of your wedding. If you're going for a more contemporary feel, don't feel obligated to choose "Canon in D" as your processional, just because that's what everyone does. Feel free to say something personal through your musical choices! The more unique your choices, the more memorable for you and your guests.
3) If you can't imagine yourself enjoying the song, don't use it! Don't feel like you have to use music that your great-aunt Gina will like. This day is about you and your soon to be spouse! Choose music that you both love!