Do sandstone patio pavers/grout need to be sealed?
While it wouldn't necessarily hurt I see absolutely no need to do this.  I wouldn't bother.


Some websites suggest it will change the appearance of the slabs?  Sealant will change the appearance, this is neither inherently good or bad.  It will give it a more...wet look most likely, similar to waxing a floor or sealing wood.
Some site also say you need to reseal after a few years - if you don't reseal will you just be back to if you hadn't sealed it, or is it a case of once you've done it once you have to keep going?  Seal, like deck stain, will have to be redone every 2-3 years (give or take, depends on your climate and how much sun/moisture the sealer gets)
We were told not to use a power wash on the stones, but the sealant guy says you can do exactly that once sealed - so it is tough stuff sealant then?  As long as you do not use too much pressure you can power wash the stones, too much pressure could gouge the stones, even with sealer you would want to be careful of this, too much pressure would eat right through the sealer and stone.