For me the answer to this question is a certain no, and here’s my reasoning:
The ‘man in Japan’ most certainly didn’t factor in the inexpensive bit of glass you are planning to add to his optical engineering masterpiece. You might think that protecting a lens front element from scratches is good, and it is. However doing so by adding extra blur, lower contrast and more flare to all pictures is for me, not the way to do it.
You will notice that the picture at the top is a very old filter, the Canon logo is from a generation long ago.
You bought a new lens to get better pictures…
For me the best option to use the lens is as the designer envisaged it, with the full optical performance working to help me produce better pictures. There are expensive and some cheap protection filters on the market, the skylight and UV filters fit in this class. They have been designed for specific use, to reduce UV light or to enhance your pictures, much like a neutral density, graduated or polariser would. However I don’t use the effects all the time.
Canon even makes some of the protection filters themselves, though you can rarely find them in stores. If you must use filters then don’t be tempted to buy a filter that costs very little. If you spent one thousand pounds on the lens, then is a fifteen pound filter going to add to it’s performance? Good filters are useful but only in situations where their effect adds to the picture.
So how do you protect a lens?
Use a lens hood. All the Canon ‘L’ lenses are shipped with them, and they are available for all the other Canon lenses as accessories. I wish they were standard in the box for all lenses though.
A lens hood does not detract from the optical performance, it stops rain and often small children’s sticky fingers from reaching the lens – particularly the expensive longer lenses like the EF70-300mm f/4.5-5.6L IS USM or EF70-200mm f/2.8L IS II USM. Critically a lens hood will reduce the instance of flare, whereas a filter will increase the instance of flare. Some years ago I did some simple tests to see for myself if there was a benefit to using a lens hood. What I found was an increase in micro-contrast in my pictures with the lens hood. There were some extra spikes in the histogram that are not there in the images shot without a lens hood, contrast had been lost without the lens hood. I’m convinced enough to use a lens hood on all my lenses all the time.
If a stone or something is going to hit the front of a lens then it will break a cheap filter and likely the glass shards will scratch your front element anyway. To replace the front element of many lenses is not as expensive as you might think. A lot of Canon service facilities would replace a front element as a fixed price repair for not much more than the cost of a good expensive filter.
Dust and water protection
Many of the Canon L-series lenses are dust and water sealed, though some like the EF50mm f/1.2L USM are not sealed against water ingress from the front. Here a filter can increase the weather resistance of the lens in poor weather.
Writing on the lens
I’ve seen reports and pictures where the writing on the front of some lenses is reflected off the filter surface and appears in pictures. EF-S 10-22mm f/3.5-5.6 USM, EF16-35mm f/2.8L II USM and EF17-40mm f/4L USM this means you. In fact you can see the text on the front of the lens reflected in the picture above. These lenses when stopped down, like you might for for depth of field in your landscapes, have such depth of field that you sometimes see the text reflected in the filter around the edge of your images. Clearly the designers didn’t want you to use a filter even for landscapes.
So when the question is asked by the local camera store: “Would you like a filter for that lens?”, ask if it already has a lens hood and if not get a lens hood instead. Then buy one of the ‘Lens Pen’ cleaners and keep it with you to make sure your lenses keep on delivering the best optical performance.
These are my views, your’s may be different share them in the comments.