The Falcon 9 first stage booster is meant to be fairly flexible. Reusability is meant to reduce cost. There is nothing stopping them from launching an expendable version in order to maximize payload, if someone was willing to pay for it.
The Falcon 9 first stage is the basis for the Falcon Heavy, where there will be two variants of the core. The two side boosters will be the same, but differ slightly from the center core booster. This will keep manufacturing costs down. The legs are attached later, and the grid fin control system is part of the interstage, so it seems likely they could build a lighter interstage without the grid fins and N2 cold gas thrusters.
In order to offer maximum payload to LEO, should a customer wish to pay for it, SpaceX has some flexibility. For minimal cost of launch, they intend to return all three cores to the launch site (RTLS). But that has the highest cost in terms of payload launched.
For higher payload masses, they intend to return the side cores to the launch site, but recover the center core downrange on a barge.
Potentially for even higher payload masses they could expend the center core and only recover the side boosters. There is only one ASDS in the Atlantic, so it is unlikely they would be doing down range recovery of the side boosters, but that can change with time.
Thus the main issue in expending the core or not comes down to, is it worth it? On a standalone Falcon 9, maximum payload will be less than a fully recoverable Falcon Heavy, so it may just be cheaper to launch on a Falcon Heavy recoverable, than on an expendable Falcon 9.
Once they started landing first stages, SpaceX made at least some kind of landing attempt on most missions. Sometimes that meant guiding it down to the ocean and letting it fall in (when the ASDS was unavailable, or due to weather). After that point, it seemed like they got it nailed down and all launches would be recovery attempts after that point.
The Echostar 23 mission (targeted at Mar 2017) turned out to be too heavy to launch and recover, even on a downrange ASDS, so will be launched expendable. Musk said that this would be the last expendable launch. Further heavy payloads would launch on a Falcon Heavy instead.