Before the actual custom home building begins you will have to arrange for temporary power to the site. You will need this in place before the framing begins.
Once the rough in is done for the all systems requiring power the electrician can come in and do the wiring. This will avoid the need for rerouting or cutting after the job is done.
The wiring for other things such as phone and cable should also be done at the same time.
Do a walk through with the electrician and mark the location for all outlets and switches.
Also ask for his input on fixture location. It is better to have an extra outlet put in at this stage because once the drywall goes up it will cost much more to make changes or additions.
Keep in mind things like possible furniture placement and special lighting or location of phones or other equipment requiring a plug.
Make sure switches and outlets are not obstructed by doors in the custom home design.
Select appliances and fixtures and make sure the outlets are accessible when the items are in place in your custom home.