To clarify: pre-knocked in means that the bat has been run through a press a few extra times in order to compress the wood fibres. In theory it should be fine to use straight away but as a precaution I'd recommend a couple of hours knocking in and use against old balls in the nets (GOOD QUALITY balls which are at least 40 overs old).
Whether you knock it in or not really depends on how long you want the bat to last, what kind of performance you're after and also the quality of the bat. Personally, I don't bother apart from using bats in nets against older balls but then again I like the 'softer, uncompressed' wood as it tends to play better.
Oiling your bat - if all natural and no toe guard then a very worthwhile thing to do. Avoid oiling the splice as it can damage the glue. Remember to only apply a light coat (dab the oil on a cloth and wipe it on the bat), do not soak the bat or stand it in oil, its better to under oil than over oil. If you do over oil the bat then you need to dry it out, a couple of days in the sun should suffice.
For most people on here I'd really recommend that they get a plastic face cover as this will really help in protecting your bat and giving it a good chance of a longer life. If you're a lower order slogger or no.11 then even consider getting a fibre glass face fitted as this will really help the longevity.
To address people who are have had problems with some bats breaking straight away, remember that a bat is a natural product. Despite the best efforts of the bat makers sometimes bats will just break - this can be down to many things and there isn't much you can do about it. This is why most bat manufacturers will replace new bats that have broken without much fuss or at least offer to repair them for free. Give them a call, explain the problem and I'm sure that they'll be happy to help. However, from my experience, I've found some manufacturers bats to be a bit 'softer' than others and therefore they do need a bit of TLC when it comes to preparation, especially in the knocking in.
As for surface cracks and seam marks, well, these will and do happen and there isn't a lot that can be done. However, bigger cracks on the edge and toe should be inspected and dealt with if possible. First step would be to apply some 2in fibre glass tape to the crack, however, depending on the severity you may wish to glue the crack . If in doubt ask your local bat supplier (or post a picture and I'll give you a brief opinion on what needs to be done (i've been repairing bats etc for close to 10 years)).
This
link gives more detail on how to care for your bat as well as providing a few pictures.