First, keep the mulch about one foot away from trunks of trees or shrubs. These measures prevent direct contact between any disease organisms and your plants. And if you pile fresh chips for a month or more before use, any disease organisms will almost certainly die off. It can result when chips have been stored in a poorly drained location or in piles higher than 4 to 6 feet. The problem is a lack of oxygen during chip decay.
Some people think that mulch attracts rodents.
No seeds, no rodents - and no rodent injury to tree trunks. As an added precaution, keep the mulch one foot away from the trunk. This won't happen to any extent unless that mulch is fresh wood or leaves. With this kind of mulch, the best advice is to pile the mulch for at least a month before use, to let some decay happen. Partially decayed or coarse-textured mulches break down too slowly to tie up much soil nitrogen.