I have had new tile installed in an entry, and have had differing opinions about wheather or not to seal the grout. One guy says its not worth the time and money to seal, and that it has to be resealed every 6-9 months. Also, the grout has to be perfectly clean before it is sealed. Another installer makes a special trip out later to seal the grout to make sure it gets done right. Does anyone have any experience/advice on sealing grout?