Nothing you do to your car actually voids a warranty. However, if you make a modification to a car that causes a problem, that will not be covered.
Safe bet is, anything closely related to what you modify is going to be a pain to get covered. You bag your box and a sway bar fails... it isn't going to get covered. You supercharge or turbo your car and blow the motor... it won't be covered. You install a subwoofer and your head unit quits working... good luck getting it covered.
A dealership may warrenty their work but other than that your just paying extra money.. if you know some one that can do it go ahead and have them do it. it wont void warrenty unless they can prove the part you change caused the problem.. and at that point wouuld only void that part of your warrent. (example if you did something to the suspension that caused problem then they may not cover suspension fixes but warrenty on motor or other areas of car would still be covered (sp?) ) make sense.