Hi lally,
The question whether to post on BS or P&L accounts depends on many things such as e.g. local accounting rules, runtime of the project and your controlling approach for projects. From a finance perspective people always try to get a match between cost and revenues within single accounting periods. This means that you are not only accumulating costs over the lifetime of a project and post revenues at the very end but that you try matching them already when the project runs. For short-term projects you will often see P&L postings and for long term projects BS postings but it really depends on the specific circumstances.
Hope it helps,
Regards,
Ludwig