I belong to the school which uses a lens shade and no filter, but my style of photography is very relaxed and doesn't involve sea, dust, crowds etc.

In general I would use a filter as a protection for the lens only for situations like seaside or hiking, where the "danger" of dust or spray water is real.

The greatest menace that lenses have to face is photographers obsessed with cleanliness of the front lens, especially those using Kodak "paper" which cannot be good enough (you can feel it's not soft).

I very rarely clean my front lenses. The detergent is typically my breath. I really have to spot some grease to use some kind of detergent.

The cleanliness of the last element, the back lens, is more critical than the front lens for image quality, but it is also kept clean more easily.

I tend to put on and off lens caps a lot. Normally I would put both lens caps on a lens before putting it in the bag.