Couldn't agree more. It usually doesn't cost that much more and you don't get a second chance at it. Also, a good pin "module" size is 15' wide to allow for two rows of pins facing each other, room for players and circulation and to take the glass off. If you have 15 pins a 15'X22' should fit 16 pins, and if you have some space, I would allow for a little bit more. Another option is the long wall option, allow 32-33 inches per pin and 10' wide. Go wider if you want to have something else like foosball or superchex along the aisle. In a walkout, this is generally better on the side buried in the ground, no windows to contend with.

The buried wall is also a natural place for the theater too. The theater space should be about 17 feet deep by at least 12 foot wide. You can go to a 15 deep but most of the home theater experts will tell you that is not deep enough (with an eight foot ceiling, so here the 9 foot ceiling will help too). Since you are building new, you might look into recessing the front row of the theater but you may find that is more costly that it is worth.

Since you are building a music room, I would consider putting an egress window in the room, so it can also be considered a legal bedroom down the line. Also because of that, if you can swing a shower stall in the bathroom, I would consider that. Minimal cost and space (going from a 3X6 foot to a 5X7 foot is only 17 more square feet). If the budget doesn't allow, rough the plumbing in now.

Finally, for the storage room, a standard closet module is 7 feet wide (2 foot shelf, 3 foot aisle, 2 foot shelf). I would put a 36" door into this space, and if you are planning on storing anything long (e.g. Christmas Tree), make sure you can maneuver it in and out.

Good luck, share your plans here when they start coming together!