This is an interesting scenario. I've done both things (written for specific people, and written stuff without anyone in mind,) and I think that the most important thing is that if you're writing for specific people, they should know and you should tell them what you're doing to some degree. You should be very familiar with what repertoire they can play well and what's their overall technical level.
I've had mostly good experiences with this as people I've written things for trust me enough to let me do whatever I want, and it's worked out pretty well. However, you can't count on that being the case and it could as well be that they can put restrictions on what they want you to write, etc etc. If it's an outright paid commission, then sure it doesn't matter that much that you cater to their wishes since a job's a job, but in my experience I've always done things in a way where I have the freedom I need to do my thing first and foremost. However, I understand that may not be always possible or reasonable.
The best way to go about it, in my opinion, is to write for no specific person and write what you actually just want to hear. Then, after you've written the piece, see if there are any comments on possible changes or interpretations that you may be willing to compromise on. I think this gives off the best impression of you as composer since you are sure of your work but at the same time you are open for suggestions, just remember that you are the boss in the end, with all the responsibility that entails.