Do I tell a potential or existing employer that I suffer from mild anxiety/depression (& does affect my work sometimes)?

Yes, you might tell your employer that you suffer from anxiety because it is a perfectly natural feeling and once in a while it might affect your work. But be sure to mention that you are working on it to reduce the effect so that your employer doesn't have a chance to judge you.

Yes I do believe its proper to tell a potential or existing employer if you suffer from mild anxiety or depression. Mainly because if its able to affect your work then your employer deserves to know this. However if you are uncomfortable or uneasy about telling them this then I think waiting a while to say wouldn't matter. It really depends on the situation though. It can vary on factors such as how your anxiety or depression affects you, to how comfortable you are expressing your concerns, especially if you feel your employer won't take the news very well.