Vincentdante
Time-Traveller
Indeed, vertical storage is bad as all the weight from the top is put onto whatever is at the bottom. It's fine for temporary storage or if they are seperated into small piles but you risk damage in the long term.Personally I would never store titles that way.
I would just add more shelves, especially if your collection continues to grow.
Source: I am a collector of omnibus (very heavy and large comic books)