In any police stop, officers won't ask you to step out of the car unless they see a probable cause to do so. Knowing this will protect you from self-incrimination.
Despite what you see on TV, police seldom ask motorists to step out of the car. And they must have a valid reason to stop a car in the first place. Therefore, police are always on the lookout for traffic violations like improper lane changes, speeding, excessive swerving, and so on.
If stopped under these circumstances, keep a cool head and follow the officer's instructions. If asked to step out of the car, do so; if not, stay behind the wheel. Disregarding his instructions may be viewed as a potential threat to the officer and may give him more reasons to scrutinize you further. Minor traffic violations such as speeding normally don't warrant a search of the vehicle.
Being asked to step out of the car happens often in DWI stops because the officer has to conduct a field sobriety test. You can refuse the test, but it won't necessarily get you out of trouble. Motorists have the right to say "no”. If, however, you feel you've been forced to comply, perform a sobriety test and, or consent to a search of your vehicle, a good lawyer will be a vital ally in court.