Yes once in the country the renewal process for a residents permit is quite straight forward. As this topic is regarding entry into China and not renewing a resident permit then the Z visa is the best option for entry into China.
As for an L visa, if you can get a job offer with a Z visa entry that is always the best option in every case. Arrive on an L visa if for some reason you have no choice and can't avoid it or if you for some crazy reason seriously trust the school offering you the job.
As for the "general rule" it is to arrive on a Z visa not the other way around. Most of the old timers here have been "grandfathered" into the system, I for one find it extremely easy to renew my residents permit or get one for a new employer but others find it very difficult.
Recently 5 new teachers at 3 different Universities (one nationally ranked and pretty famous in China) were refused residents permits because they had not entered into China on a Z visa. Even going to Hong Kong was not an option and they all had to return to their home countries for processing (as a result some refused and there was a mad scramble to find new teachers)
The regulations have been in place for a few years now but time varies to how long it has taken for most places to actually enforce them.
My city (Chongqing) was famous for ignoring the rules and granting residents permits freely up until last year. Now the rules are strictly enforced which has caused a massive headache for the teachers and schools here who where completely oblivious as to how to get a residents permit through the normal procedures.
Teachers that have been here for a few years and never had a Z visa for entry probably never need to worry about it. As for new teachers it is a concern and quoting the experience of the past is not really helpful to them. As China develops and grows the visa restriction HAVE become more steadily enforced across China and as such a Z visa is highly recommended for all newbies as it's the only way you can assure yourself that you can even get a residents permit. Otherwise you are relying on the word of your employer, who seemingly doesn't have enough clout or can't even be bothered (if he can get one in the first place) to spend enough energy to get you a proper entry visa.