this is an interesting topic to see. i come from power grid/funkenschlag, which is a game where 4-player is the most popular competitive format, and the top players at BSW have a comprehensive set of unwritten rules when it comes to kingmaking. for those familiar with that game, they're as follows:
- most obviously, don't end the game early if you're going to lose. people mentioned earlier that dominion is a rare game where this is possible, but i think it's a lot easier in pgrid. in particular, you'll often see newer players do this accidentally in 5/6 player since they don't remember the lower ending condition. but then, 5-6 in pgrid is equivalent to 4 in dominion =P
- don't buy out another player's resources at the end if you can't win, unless you actually need those resources to run a plant. if it comes down to that then they're being dumb and have nobody to blame but themselves.
- don't bid other players TOO high on plants they need at the end if you can't win, but don't let them have those plants for free either. this is the trickiest issue, but generally you should bid until the plant becomes equal in cost to other available options at the same capacity, OR (if it's an especially godlike plant) you can't build enough cities to make full use of the plant.
- on the final turn, only build as many cities as you can power, and take your cheapest builds. anything else risks blocking others without benefiting you.
- don't break any of these rules even if it will get you second place instead of last. BSW only tracks 1st place finishes, so that is all that matters to the top players.
basically the general concept is "do what's best for you as if the other players didn't exist". this leads to things like being able to get away with not storing resources on a plant, because nobody can buy you out without throwing the game to someone else...so it's definitely not perfect. i think it's the fairest way to handle this though.
but i'm sure now yall understand why i'm eager to learn a game where nobody cares about multis! =) i think pgrid is as good as it gets for a serious multiplayer board game (only caylus seems to compare from what i've heard), but i just think the format inherently can't be as competitive as 1v1 since you always have these issues along with others (e.g. how badly the game can be skewed by one lower-level player being involved).