I find that both texturing and model making both eat up enormess amounts of time. Depeding on the details on the model texturing can take longer to do and usually ends up in the biggest compromized for looks. Either way I tend to work only on my own projects, and help refine textures for others and do a vary small amount of requests.
I don't see the problem of skipping requests at all, as long as you at least reply to the requester, since not everything that comes along will interest a person. Lets face it the amount of time that goes into creating a ship with textures is a time consuming undertaking. Also if you are not behind the project 100% there is greater odds that you will not put forth your best effort or will drop it if something else comes along. I know about dropped projects because I have done a few of those, have a HDD full of incompleted works.
All I can say is don't worry about it, do what you can and more importantly what you want. It is your time and you have to decide how you want to spend it, and on what is best for you.