cost estimation

Hi All

i have a scenario pls advice me

we wont do costing run we will use actual cost as a standard cost and maintain material master

and in copa, cogs will be calculated with standard cost

1)Does this cause any errors when billing if we do not do cost estimation in material master?

2)when maintaining standard cost of material master, transaction MM02 and MR21 makes difference?

3)Are there any drawback in not doing costing run?

Thanks & Regards

phaneendra