Things to be kept in mind while evolving pokemon in pokemon go?

If you evolve purely to gain experience it doesn't matter what the pokemon stats are (using lucky eggs while evolving many pokemon will help you gain even more xp).

If you aim to eventually get the best possible pokemon of that kind, lets say you want to have the best Raticate possible, you want it to have as good Individual Values (IVs) that compose of your pokemons attack, defence and stamina, that aren't visible in the app, but using an IV rater you can see how good these values are (just google 'pokemon go IV rater'). Also better raters show all possible attacks that a certain pokemon can have (usually values of power/second given). You want your best pokemon to have the best attacks (this might be even more important than IVs). Also if your pokemons attacks normal(fast) and special compose from different kinds (Steel, Water, Fire etc.) they might be more diversely better against other types. So if your pokemon attacks only in with Fire attacks and you encounter a Water pokemon that resists your attacks then you might wish that either the normal or the special attack would be of another kind.

My personal tactic is that regarding a pokemon family (e.g. Charmander, Charmeleon, Charizard) I keep pokemon with good CP until I catch, train or evolve a better one. I never send a pokemon to the professor unless I have the same pokemon or a pokemon from the same family that has better combined IV + attack set. Also just to feed my inner (but minor) OCD I tend to keep one Charmander and Charmeleon even if I have a good Charizard measured in CP and IV.