Probably several times during your career, you'll have to break glass (e.g., in a window or door) to gain access to someone who needs help or get at a bad guy. There are several types of glass, and some, like the chicken-wire fire glass in schools, is not feasible to try to get through in any short amount of time. Safety glass, generally found in doors and windows that reach to the ground in commercial buildings, can often be broken easily and safely with a window punch you'd use on a car. A window punch should be available in your cruiser or first-aid kit. To be safe, however, carry one in your personal gear bag as well. It will cost you only a couple dollars.
Regular glass, which is what you'll commonly find in residences, obviously shatters easily. But almost every officer I know who's broken this kind of glass has gotten cut either a little or a lot. The tricks to not getting cut: 1) Use as long a tool as possible to break the glass; 2) make sure you run your breaking tool along every side of the frame several times to get all the shards out of the frame; and 3) wear glasses and gloves, and cover your neck, because shards will fly and find these exposed areas. Finally, consider throwing a heavy object through the window to avoid being near it when it breaks and those shards go flying.