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.
Tile Doctor Tip: The doctor always recommends sealing cement grouts. For ease of maintenance and protection against stains, this is a small cost of the total job, but the affects of not sealing can make you unhappy with the whole job.
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.