If you do accidentally hit a live electrical connection, at least you won't be grounded through the drill - and possibly shocked. If you hit any obstacles while drilling or cutting, stop immediately until you figure out what you've hit. Be prepared to get dirty and maybe squeeze into spaces barely fit for a human. After you've found the best speaker locations and measured and visually rechecked everything to ensure proper orientation, affix the speaker templates. These are flat cardboard or paper guides that come with the speakers to indicate the size and shape of the holes to be cut.
Leveling is extremely important.
The eye is drawn to any deviation from right angles and parallel lines. You don't want to step back, after doing all that work, and find you've mounted a speaker cockeyed. However, keep in mind that your walls or floors might not be exactly level, so you may have to touch up the adjustment by eye to keep the speakers visually parallel to the wall or floor. Use the stud finder to double-check the location of the nearest studs after you affix the template. Then drill a small hole or cut a small slit in the center of the template.