Good question. I have also been holding onto all viagra since I started this game a couple weeks ago.
I think it would be a good idea to spread the resource evenly since it seems like to do well in events like colosseum and world bosses you will need more than one set of card, and the world boss portals specifically requires you to use cards from a specific element.
If you put everything into the same element your cards from that element will be very powerful but you likely won't do as well in the aforementioned events than if you spread. Also if you are putting everything into the same element you are likely running decks with card from that element only. I don't know if it's a good idea because if you run into opponent with the perk that allows gem of other elements to drop more frequently, you are pretty much screwed. You probably will have a hard time beating enemies your size in the tower if you invest all viagra into the same element and your gems are not dropping.
I am only level 18 in league 4 and have not used any viagra yet. I occasionally beat players that has 2x more hp that mine if they only have cards from the same element. I run a 1yellow/2purple/2green set up with the green gem drops more frequently and purple damage 100% perk. My green cards have ability that synergizes with my purple cards. I think eventually I will spread the viagra evenly.
Would love to hear what the pros say.