Nah I faxed my order into OAT before my PPL had hit the door mat.
You can use hour building as a treat for doing your studies.
After the next 5 sections i can have 10 hours flying etc.
You have to have finished and passed you ATPL's before any skills test and you have to have built 150 hours before starting your CPL course. You are actually better saving 10-20 hours of you building until just before you start your course. Then you start current. Would recommend though that you have all the exams out the way before starting either IR or CPL
MJ