With a fully tunable 3D map you can get the proper absolute timing regardless of the base timing you choose. The problem is, even if you don't set it, the base timing setting exists...it's there...you just don't know what it is. Every time you mess with the cams it changes a little and thus changes your absolute timing values. Setting the base timing means that your map always corresponds to the appropriate absolute timing value. If you don't set it, then it can be different from one setup to the next...as for instance when you switch from OEM to 264 cams.
Anecdote: I pulled off my CAS to replace the o-ring and then reinstalled it "by eye" to where I thought it belonged. The car ran LIKE sh*t. Barely idled, sluggish and lurchy, and acted like it had fuel cut at anything above 10psi. I pulled out my timing light and found that it was set at 5* ATDC. That's 10 degrees of retardation. You can barely see that from the orientation of the CAS but it makes a HUGE difference in how the car runs. Granted, I could have put in and EMS system and systematically added 10 degrees everywhere (probably requiring a day on the dyno to experiment and figure out how much to add and where) but it was a simple fix to just pull out the timing light, set the base timing, and then KNOW that the timing maps all meant what they say.
As a final note, remember that the maps report the adjusted timing value (with the base setting already factored in) so if you want what the map says to correspond to what you actually get, then you should set the base timing where it belongs for consistency.
Lastly...and I don't mean to be picking a fight here but...you're supposed to be a professional. And you're charging people money to work on their cars and you don't know how ignition timing works on a 4G? I must say that I find that appalling.
