Warning: Cheap Filters Can Harm Your Glass
There is quite a debate going on about whether one should use filters for lens protection or avoid filters for better quality.
Here is another thing to take into consideration - Some cheap filters may harm your glass, not just your image quality.
This got Roger to experiment with various filters. He realized that the cause for scratches is contact between a cheap "thin" filter and the slightly bulged front element of the 24-70.
The good part is that not all lenses and filters produce this undesired effect:
"It seems that you need the proper combination of a thin-line filter with glass close to the back of the filter, and a lens with a slightly bulging front element (this lens has one, but so do lots of others) to even worry about it. I would also think that wider front elements (this is 82mm) might allow more play or sag in the center making this more likely" #
Rojer suggests to "stay away from ultra-thin filters on these lenses, especially discount ultra-thin filters" or at least check that the lens does not come in contact with the filter.
[Bad Times with Bad Filters | Lensrentals.com]