Stacking plastic trays of chips? Page 2. Discuss Stacking plastic trays of chips?, on ChipTalk.net the place to go for your Poker chips and gambling tips. Read it in Poker Chip Care.
As far as hurting the chips, if you get good racks then you have a nice even pressure on all 100 of the chips from the ones above. Since the chips are already on on their sides, they're solid as a brick IMO.
probably gotta be careful with the cheap acrylic racks from china. the last chips in the row do not lay flat they are raised up a bit. if you stack another rack ontop, this chip takes on most of the pressure and could cause problems. stick to paulson racks or chipco racks. there was a thread in poker gear dealing with the topic, ill try to find it...