I think placing and upgrading evolving buildings is one of the most important things to do.
Evolving buildings, even the crappiest of the lot, are typically better than any standard building. Why? They provide a lot of "free" resources, in addition to the population and culture benefit. I'm sure you're saying "well Hawk, the free resources really isn't that much. Who cares about 6k t2 goods every 24 hours."
OK, at face value, you might be right. However, have you considered the cost of your goods from the factories? Each factory requires a huge amount of population, culture and supplies to produce goods. Think about how many "support buildings" that are required to run a single factory. Likely it's 2 or 3 residences, a few culture buildings (to support the residences), workshops for supplies, and all the roads to support all those buildings. In reality, your 16 square factory (or whatever the size really is) takes up over 70-80 squares to run it. (if you want accurate numbers, go read the forums, lol)
Back to Evolving Buildings. Any building that produces free goods, free population and free culture, is actually freeing up space in your city for other things. In the example I mentioned, the 6k t2 goods is approx. 25%-30% of daily production of a factory. Place 3 evolving buildings, you can then eliminate a factory AND ALL the support buildings, and save 50% of the land...
Now, once you become reliant on the evolving building producing a certain level of good, you may not want to upgrade it because it some buildings switch to seeds or mana. Make wise choices on what your production needs are for a balanced city. Additionally, there are a small handful of evolving buildings I do not upgrade, Fire Phoenix being one of those. I don't need more coins lol!