Woops! I kept writing ‘b&h’ filter while it’s a b&w filter, sorry.
Have to agree with all ya’ll, – it’s a judgement call for each person to make, based on their preferences and a scale of diminishing returns (ie.: 200$filter on a 50$ lens) There is no front filter on our 50mm 1.8 or our 55-250. I personally have not ever had a problem with filters causing flare etc, but I’m not using anything extremely wide, and always with a lens hood.
Realize I made a mistake with those test images. I should certainly have focused manually to give a level playing field. I just looked them over again, and I agree with Timtam, in that the second sure looks a mite softer. Should have read this on autofocus accuracy: http://www.lensrentals.com/blog/2014/02/why-you-cant-optically-test-your-lens-with-autofocus
Which means that my most rigorous technique is limiting my iq more than the filters.