That is correct. You can use an inexpensive sealer to pre-seal the crazed tile prior to grouting. Without pre-sealing, the pigment from your grout (even white grout) will penetrate the glaze and leave a residue behind. If you soak a mat of paper towels with the sealer and wipe it over the tile you should get a thin and even coat. Just remember to polish the tile clean while it's still wet with the sealer to avoid streaks. Normally, I apply sealers with a pump-spray bottle but in this case that would also get it down into the joints and you want to avoid that as much as possible as it would interfere with the grouts bonding. So, just wipe on, wait, wipe off before it dries. Do small sections at a time to maintain better control of the drying time.
Best of luck,