Particularly in tournaments, dice etiquette is very important. It is one of the key things that can lead to charges of bad sportsmanship but is one of the easiest things to ensure you get right.
Below is a list of things that I've picked up over the past decade or so of playing tournament games:
1. Always say what you are rolling for first - pretty self explanatory but absolutely vital. Rolling a dice and then saying "Oh that was for my ward save/lascannon etc is guaranteed to rile your opponent. The easiest thing is to be open with what rolls are for before you roll.
2. Always roll your dice where you opponent has a clear view of the results - regardless of which game you are playing, it is a social contract. You owe it to your opponent that they can see the outcome of any role. I don't want the uncertainty of dice rolls behind terrain etc.
3. Be consistent - nobody likes an opponent who is inconsistent in their application - taking any benefit but rejecting any negatives. Therefore it is important to establish any conventions you use upfront - and then consistently apply them. For instance, if I roll too many dice I will pick them all up and re-roll them regardless of the result. Whereas if I roll too few then I will pick up the extra required dice and roll them. Similarly, establish a convention for "cocked dice". I reroll any die that is not "flat on the green". That means it needs to be on the table flat - not on terrain, not on a movement tray, not on a book, piece of paper, cards or army list. The key is to be totally consistent in the application of that convention.
4. Pick up "Misses/Fails", leaving "Hits" - there is a reason for this....it leaves no uncertainty. If you pick up hits then there is the chance of mistakes whereas if you leave hits and remove misses, that uncertainty is removed. I always leave the resultant hits and ask my opponent "Looks Good?" Before I move on.
5. Symbols on one face only - with the rise of customised - or "club" dice - there is an opportunity for confusion if the markings are not consistent. Using Skaven dice with symbols on the "6" for some whereas they are on the "1" for others is sure to rile your opponent. Stick to symbols on a single face to ensure there is no confusion.
6. Dice Use - the default is that you bring and use your own dice. You should never touch your opponent's dice without their permission. This is just general politeness.
7. "Special " Dice - wargamers are a superstitious lot - as well as being strange. One gamer at our club used to identify non-performing dice, then line up his other dice to watch as he smashed the non-performers with a hammer. I have a set of "Strawberry" Chessex dice and I swear that they roll unbelievably well or unbelievably badly - never consistently average. They are just too "scary" to use.
8. Opponent's Rights - sometimes dice may get on a "hot" run. In a tournament situation your opponent always has the right to ask to ask to either use the same dice as you or that you use a different set of dice. As a tournament organiser I never want to get to the stage whether table dice are required but that means that players must be willing to accommodate opponents when asked.
9. "Hot" Dice - it goes without saying that if you suspect that a certain set of dice or an individual dice roll consistently well then you should definitely not use them. Dice are a vehicle to introduce chance into a game and if you suspect that your dice are skewed to a particular set of outcomes then it is clearly unacceptable to use them. This is particularly true in a game like KoW where it is always desirable to roll high.
So it general boils down to three key planks - consistency, openness and fairness. Most issues can be resolved by discussion or by both players using the same dice if there is any suspicion of dice being skewed.
From a TO's POV the last thing he wants with his event is controversy and once a player has aspersions raised around their dice etiquette it can haunt them for a long time. To this end the best solution when playing any wargame is to be both flexible and accommodating.