Grout Sealing:

Always follow manufacturer’s instructions on the specific sealer being used.
Some sealers breath naturally so a "still wet" installation can be allowed to dry out even after the sealer is applied. However, some sealers lock in the moisture as they lock out the stains, so make sure which type of sealer you are selecting and the proper technique of application.
Your choices in grout sealers are a topical sealer that can offer a wet look, or a penetrating sealer that has a natural look, which protects the grout but does not change the look of the grout.
When sealing the grout there are various techniques to apply the sealer.
Always follow the manufacturer's directions, but generally the best methods are to apply only to the grout joints and buff off any excess that happens to get on the tiles.
Another method is to apply all over the surface with a sponge or cloth and then buff off the excess with a terry cloth or cheesecloth rag.
Some sealers protect against everyday dirt and minor staining elements; others protect against harsh staining elements like hot grease. Look for warranties and protection information on the label from the manufacturers.

A little time spent on this can save a lot of grief later.

Grouts can also be re-colored by applying topical coatings much like painting. There are special products made just for this technique and the best news is, they work! If you have damaged grout that is discolored, you can fix it.

Sealing Tile

Always follow manufacturer’s instructions on the specific sealer being used.
Stone
The first step in stone tile maintenance is the sealing of the stone. Generally, all stone must be sealed. Follow the manufacturers recommendations for the product you choose.
Know what the surface preparation (polished, honed, or natural) coupled with the density and porosity of the stone to determine the best type (petroleum or water based) of sealer to be used. The other consideration is that you use the best product the budget will allow.
Ceramic
The first step in maintenance is the sealing of the tile (if necessary) and the grout (if necessary). Generally, glazed tile requires no sealer. Some unglazed tile requires no sealer. Know the type of tile you are installing. Test it with a small amount of water. If the surface darkens, it absorbs water and needs to be sealed.
Generally, cementitious grouts require sealing while specialty grouts like epoxy and furans do not. If the grout darkens with water, it needs a sealer.
Glazed tiles should never be sealed. The glaze is the "sealer" and is far more permanent and resistant to wear than any sealer.
Many people seal their tile that is installed indoors to protect against everyday dirt, stains and wear. When sealing tile indoors a topical sealer is important in some types of softer tiles like saltillo (topical produces a surface coat). When using a topical sealer, a "wear layer" or "sacrificial coating" of acrylic floor finish should be applied in addition to the sealer. This "wear layer" of acrylic floor finish will keep you from wearing through the sealer and having to strip and reseal the entire floor.
Some manufacturers have sealers that incorporate a sacrificial coating together with a penetrating sealer. This can be the best of both worlds.
However, it is critical that you maintain this wear layer.
On indoor installations of hard, dense tiles or stones, many people select a penetrating sealer only. This type of sealer leaves no topical finish and therefore does not require a "wear layer" coating, but will give lasting protection from everyday dirt and stains.
The use of sealers on tile is an area that books could be written about, with all the options available and their relations to each other. We speak only in very general terms and keep in mind; there are exceptions to every rule. Manufacturer’s instructions must be followed closely after a sealer is selected. Please do not mix brands. Once you have started with one brand, do not apply another on top of the first, hoping for the proper results.
When in doubt always call the manufacturer for specific details. Retain the information on who you spoke with and it is a good idea to have them fax you or email you the instructions to fall back on if there ever are any questions.

hope this helps some newbies questions on sealing grouts and tiles ........dave..