If you blow the BCU with aftermarket lights whether installed "properly" or not and the dealer discovers it they can and likely will void warranty. If they are installing OEM fogs then no it doesn't void warranty since they are approved parts installed buy an approved service provider.
As I said before there are many examples on here of the BCU blowing even by people who swear everything was installed properly. Part of the issue looking for the threads is that many bad BCUs had symptoms other than just the lights and it makes things confusing.
I am not saying for one second that you should not do it but you do need to be aware of the consequences if you do blow the BCU.
http://www.google.ca/url?sa=t&rct=j&...EwE1Dt4TMgq-iA