I think with all wargames this is left to individual groups of players to decide, just because few people agree to one set of rules for this, and everyone's terrain is different. Also an official rule might be very long to include all possible situations?
For trees, I use actual tree models and assume the whole wood blocks line-of-sight to that height. Before I made some trees, we used to designate a set height equal to one of the smaller buildings. Same goes for hills.
Pinning class is a simple way to decide- we generalise infantry as not blocking site to everything else, then make-up the rest as we go along, roughly based on where the weapons are on models and heights and stuff- so lots of arguing. What's wargaming without arguments??
Clearly arguments are a staple of the genre!! If in doubt, I think it's best to be generous to the player firing- seems to work out for the best. Or you can roll a dice for it. I expect that answer won't satisfy..
be serious!
