CSS clearance is definitely a luck. People have different experiences. Mine is a little different too. I got the job in 17 and forgot the competitive examination due to job requirements, despite I loved to appear once in a lifetime even without preparation. That depends upon you, how you spare time for such examination. Following post might be helpful for you.

1- If you want an opinion based upon rules and regulations than you won't be permitted to appear in CSS exam till the completion of training. There should be a service bond devised by NAB for two to three years service after training. Once you completed the service period, you will be eligible to apply for NOC/departmental permission. If you want NOC during bond period or in case of quitting the job, you will submit in writing that you'll payback all the expenditures incurred on your training including other expenses. In simple words, forget NOC. Bypassing above will put you up into litigation or departmental action.

2- Rules and regulations for powerful people are much more flexible and different in Pakistan. The term powerful contains many characteristics. e.g Strong financial background, Strong nerves, Strong political, bureaucratic or personal relations etc. One should have a few qualities to come into above mentioned category. People of that kind join the organization, become absent or avail leave for one or another reason, appear in css exam, throw out the resignation and disappear. They use their skills i.e. power to tackle the issues.
Now, it is up to you. I mean you have to decide before taking any decision.
Remember one thing.
A bird in hand.........................can make your stomach filled.