We have also sealed all the floors (ie. grout). It does help against wet stains penetrating and settling (but I do wipe the spills as soon as I see them). I am not sure if there would be any difference if I didn't seal all grout . But, as I was sealting them, I noticed that the second coat was barely able to penetrate at all because the first coat apparently already did so! So I guess the same happens with spills ...
Another thing is the cleaning technique and its frequency. I was initially swiping the floors with soft broom, but noticed that dust gets into the grout lines (which are slightly lower than tiles), so then moved on to vacuuming. I now need to clean the grout because it's gone grey due to ??stuck-on dust??
BTW, please don't clean it with bleach! Unless you are quick and careful, it will discolour your tiles. I use cream cleanser and it works well for me (so far). Selina, what do you mean "they look grubby"??
Spot on Lex! Wipe it dry before letting it dry out, dry sweeping is good but as you said vacuuuming is alot better before wet cleaning. When applying a clear sealer it is best to wait 1-2 hours before the second coat to let the first coat deeply penertrate and it all depends on the quality of the sealer and what type of sealer water base vers solvent base. With solvent base clear sealers they penertrate faster that is the only difference.
You are also spot on about bleaches they will also discolour the dyes in grout after long periods of use. If you really want your grout to come clean again use a high alkaline cleaner and scrub with a grout brush then rinse, here is a short video of using a high alkaline cleaner. http://www.groutperfect.com.au
Craig