dice can easily be weighted to land on certain sides...especially if you used lead paint to fill in some of the pips, that can weigh a die to roll on certain sides...and yes, I'm talking about 6-sided dice...which are the easiest, because they have less sides and being the sides have more of a base than other dice (except 4-siders, but who uses those outside of RPGr's)
unless you're catching your flips (which is not really allowed during coin flipping), the surface that you are playing on should provide enough bounce so that it is still fairly random which side the coin lands on...especially when there are playmats (most playmats that are used these days, that is) being used, and considering when flipping you are only supposed to be allowed to flip pokemon coins that have been unaltered (anything in the grooves of the picture, etc.), there shouldn't be an issued about using weighted or trick coins