If you're gonna do a BTO Macbook Pro w. Retina, upgrade the ram. It really depends on how long you'll be using the laptop, but overall future proofing is best. Why?
The RAM is soldered onto the board. You can not upgrade the RAM by yourself. Older Macbooks allowed you to buy third-party RAM modules so you can DIY, but the new notebook won't allow you do that. Same goes for the storage, although that can always be resolved by a USB3.0 or Thunderbolt external HD/SSD.
It sucks, and yes, memory upgrades at Apple are a total butt reaming, but because of the lack of third-party options, you have no choice but to upgrade. It's good to future proof when purchasing while at the same time it retains your resale value.