how old are you Ermin? live on your own yet? economics will soon enough force you to either shit or get off the pot... that is to say, take your career more seriously and work for whoever hires you (given that you like the music and pay) and let the "moving the music business forward" bit to the market, which will drive it no matter what you do with your career. besides, some clients.. whether they are signed or not... have a mastering engineer in mind, and while you may want to offer them your version as an option you cannot/should not try to talk them out of it. and you certainly shouldn't be demanding or dictating that you master.... had that accusation made to me once already, even though it strictly wasn't true, and it's no fun.
in my situation i agreed to mix a project for X amount of money based on a belief that i was helping a new buddy who was tight on cash.... then i found out they were going to be paying a mastering engineer over double what they had paid me to mix.... i strongly suggested that i master, and added that if they could pay that guy so much, then they could have paid me more (basically i felt i had been misled). Seemed to go over ok at the time, as they agreed to it... but it bit me in the ass in the end. i don't suggest it.
also, you never know what will happen with an indie band once you've done their album.... if you've produced/mixed such a band that then goes on to get signed to a big label, well that's out of your hands.... and what if that label likes your mix, but wants a big name to master... but you've mixed through your mastering chain. This happened to me... and it cost me the mix in the end. Again, i strongly DO NOT suggest this. always, always, always, always make your mixes fine withOUT a mastering chain in place... or you will pay for it, eventually, irrespective of any altruistic wrapping you place around your argument. Do not assume you will be given the opportunity to remix, even if you offer it for free. trust me on this one.
finally, if you are suggesting that you would turn down big label work over your principles... well you must be independently wealthy so feel free to give my contact info when you turn down that work. i'm not buyin' it though man... it all sniffs of a bit of self-serving justification to me, and i've done enough of that myself in the past to recognize the smell.