In the default AI .txt files, there is absolutely no difference between a tribute and a gift regarding chances for accepting/refusing nor happiness modifiers. There may be differences for individual races.
General communications with the AI are useless and in some cases (like Xenophobes) it actually makes them angry.
When the AI makes a demand/request, accepting the demand and not following through still makes the AI happy. Not answering at all has no negative consequences. To get a more full gaming experience, do not use these exploits of the AI. Answer the AI's requests and follow through to do what you agreed to not exploit the AI.
In multiplayer games, the in-game communications are much more useful and sometimes, if agreed to ahead of time, email can be used once players have "met" in game.
| Manual (SEIV)