Mary, it is not rude at all to have open seating with a few reserved tables for family and relatives. However, assigned seating provides a sense of organization and less question for your guests. There is another option, if you don't want to make place cards with a seating chart you can have table numbers indicating which table number your guests will sit. To be creative you can always label your tables with your theme. For example, if you have a beach theme the tables can be names of beaches vs one, two, three... On a separate sheet, outside of the banquet area, note which table your guests will sit. The other option for you is have open seating and have paper tents or signs tied elegantly behind the chairs indicating 'reserved'. To insure this table is reserved for your family make sure to communicate this request to your wedding coordinator and your family.
My husband is a wedding photographer and not only have we seen family tables at many weddings, but set them up for both our daughters' weddings too. The most simple way is to simply label the tables 'Bride's Family', 'Groom's Family'. Most of the time, these tables are the first tables in front of the wedding party table or the bride and groom sweetheart table, so that the family has the best view of their son and daughter during the reception. If the families are large, you simply label the appropriate number of tables. Personally, here in Arizona, I've heard lots of negative comments about the 'assigned seating' plans for all the guests. Some people come to a wedding and they want to sit with those they know, others want to sit in back because they have kids, or have to leave early etc....so we don't see that often in our part of the country, or at least not at weddings we've photographed or attended anyhow.
Have tables that look slightly different with a Family sign posted. Have placecards for the family members. On the other tables post a Friends/ Guest sign. It's your wedding, just like Burger King - have it your way. Hope your days is spectacular.