There'll be occasions when a dice will end up not lying flat – normally because it's got caught in a wedge of terrain or propped up against a unit. Such 'cocked dice' are a touch inconvenient, and certainly they seem to bedevil important rolls more often than trivial ones. I find the best way of dealing with a cocked dice is to re-roll it if the result isn't clear. Other players prefer to re-roll any dice that isn't totally flat (thus avoiding any subjectivity) or eliminate the problem altogether by making sure they only roll dice on a totally flat surface.