For re-grouting the bath shower, is there any additional sealing operation (like using sealant) after applying un-sanded grout on it?
All Portland cement based grout benefit from a sealer. I prefer the distillate based sealers, but I hear good things regarding the newer water based sealers.
One grout type doesn't need any sealer: 2 part epoxy grout. A bit more care and effort during install, but nearly bullet proof afterwards. Shower floors and foyers really stay clean.