I had volakas installed in my house, and did not encounter any problem the last 8 years. Basically a good marble specialist will precoat the marble a day before with water proof coating. If no water proofing is applied, moisture from cement base may seep through the marble leaving an ugly water mark stain.  All marbles are porous, but wont leave any stain if you wipe them immediately after spillage.

Also remember to ask your marble guy to prelabel your marble block before cutting in the factory, so the marble specialist can arrange and glue the marble accordingly, like before they cut the block. The grain will then flow nicely. Be prepare to pay a premium for this type of service. I paid Rm8 psf for labour.