For most of my photographic life when taking pictures of people (or any other subject for that matter) I've said I never use direct on-camera flash, especially un-diffused on-camera flash. I mean why would I, it is to harsh and makes the subject look flat. And what about the unsightly shadows or the possibility of red-eye. But I've changed my point of view on this. I've been reading Joe McNally's "Hot Shoe Diaries" and I must admit now I can see myself using straight on camera flash. I still don't think I would use it for a formal portrait or when shooting a wedding but perhaps at -1 or -2 it can be used as a fill or to create edge or drama. So now I can't wait to try it and experiment!
Another rule I've lived by for years was to use the lowest iso possible. Perhaps this stems from shooting Ektar 25 print film in my Mamiya 2 1/4. I do/did consider myself a landscape photographer and I always believed your safer with the low speed film if you want enlargements. This low iso rule has it's place but I found I was kind of hung-up on it so much so that I would rely on it perhaps in lieu of common sense, I mean consider this photograph.
I'm taking pictures of slow moving grazing horses and I'm using a shutter speed of 1/8 second. I mean I'm using a good tripod and ball head I should (might) be able to get "the shot". Not this time, movement detected. Here I am at iso 100, if I had to go to iso 400 I'm up to 1/32 of a second and bingo I got it. Or maybe iso 800 and 1/60 second. I am happy to say at least I learned something and that should keep me from making the same mistake again.
I think it is important to remember that all of these "rules" have their place. But photography is such a broad subject and is changing so rapidly that our concepts and ideas have to change. Sometimes in the heat of the moment when a great photographic opportunity raises it's head they might get you that great shot but sometimes it might get in the way. Well in the least if we don't get THE shot hopefully we will learn something and be more open next time.
I am amazed at how many photographers are so ingrained in doing things one way and one way only. Even people who need a lot of help!
Do you have rules you used to go by and changed? If so can you share with us and lets us know why you did change?
All the best and don't forget to break your own rules!