The only time I have noticed the difference is when photographing through glass using my 50/2.5 macro to take photos obliquely through a London bus window. There's always an extra chance to pick up extra reflections and artifacts with extra glass ahead of the front element. A hood may have helped, but the lens doesn't otherwise need one because the front element is so deeply recessed. Apart from ND and orange filters for b&w film, I haven't used a filter on that lens since.
On my only weather sealed lens, I soon bought a Hoya HD protect (not UV) filter permanently in place. I can't fault the IQ with or without filter.
6D, 5(film)x2, 650(film)x2, Canonet QL19, EF 15/2.8, 24/2.8 IS, 50/2.5 & 70-300/4-5.6L
Fotodiox Pro OM/Eos adapter, Olympus OM-10, Zuiko 24/2.8, 50/1.4 & 135/2.8, Ensinor 28/2.8, XA/2, Mju 410
Apple iPhone 6 & iPad 2. I've only listed the equipment I still use for photography!