Yes, it's totally preference and what you want to play. There are different formats where only certain "blocks" are allowed (for instance, "Towers block" is only the Towers sets 4-6, while "Towers standard" is sets 1-6). In informal play, especially as you're learning the game, you may as well mix up all the cards you have (while using the site path from one block). Limiting the card pool to certain blocks just provides an interesting twist on the games for those who like it.
Let me know if you have any other / clarifying questions. Hope that helps.