Undefined variables

Hi, newbie here. I was erroneously using "error_reporting=E_ALL ~E_NOTICE" on my debug server. Now, I changed it to "error_reporting=E_ALL" and I can see lots of warnings about using undefined variables. So, if I don't want to see warnings all over the place, I have to check every variable with isset() before I use it. Is it necesary that I do this all the time? Why can't I trust on the "emptiness" of uninitialized variables? I'm using "register_globals=Off", so there should be no problem with that. I know it's a good practice to declare and initialize all variables, but I need my scripts to run as fast as they can (and I'm too lazy to check every variable before I use it, too :-) So... do I have any choice? Regards, Ernesto