I got used to ask my clients to sign a model release when i photograph them. But not always I am saying that I will put their photos on stock. I am saying that the MR demonstrates that i made the pgotographs and that it is possible to use the photos at diffrent projects of mine.
Most of my clients doesn't know what is stock photography.
I guess it depends on your morals. If these clients are paying you for your services, then it's shady. If you know you are going to be using those pictures for stock, or think that you might, you should tell them up front. Is it really YOUR project, or selling them to someone else for THEIR project? If a client called you and was angry about seeing their picture on someone's website, how would you handle it? While every person has the responsibility to read what they are signing and understand it, they also have trust in you. Being shady abuses and betrays their trust by withholding your intentions. Just my opinion.
For client photos (They are hiring me to take their photos for their needs) I do not have them sign a stock model release, just a separate one that allows me to use them for my business). If someone mislead me to think they would use them in their projects (advertising, maybe enter in contests or use in magazine articles about them/their own business) and sold them as stock I would be pissed. Not everyone reads the model releases.
I clearly explain when I want the stock photo model release what types of things they are usually used for. I answer any questions they have about it. Most think it's cool their images can be used in magazines or billboards and such and have no problems with signing it. A few don't feel comfortable with it... that said, I'd rather not ruin my reputation by upsetting someone in that way... honesty is the best policy IMHO.
I think it's a good idea to be as honest and open with models as possible. If you are going to sell the images as stock, they should be told. If they don't know what stock is, take a moment to explain to them. I keep a stack of in-actions of other models in use (magazines, book covers, newspapers, brochures, etc.) to show them so they get the idea.
Not only is it ethical to be up front about your intentions, it lessens the chance you will be sued. The release is supposed to protect you, but if you didn't fully explain the release and possible uses, it might still leave you open to legal action.
There have been cases where models sued the photographer and/or agency because they discovered images being used in ways that made them unhappy. And that's despite a valid model release being signed.
An image can potentially be used for anything. If someone discovers their face being used for a political cause, alternative lifestyle, or any other hot button issue they disagree with, you may find yourself being served with papers.
Some Contributors have a silver tongue and can easily get strangers to sign a release. That could be asking for trouble. If you're paying models to model for you, you have more leverage, there should be no mistake the model is giving you the right to use their image if they're being paid.
Tell them up front...don't hide it away. As already stated that they might find themselves promoting a lifestyle contrary to what they believe in or stand for and the use of the image might even have financial and personal implications.
I always point out that I can't guarantee that the photos won't be misused but that it is unlikely and so far everybody has still signed. It's much better to warn them and risk losing a model than have problems later. The ones that refuse are the ones that would have caused the most problems for you if they found a misuse so it isn't worth taking the risk.