According to a Zeiss/Contax camera repair guy:
"The best way to keep your lens clean is to purchase a UV filter and then to clean it when it gets dirty. It is relatively inexpensive to replace a filter if it should be scratched. In most cases it is prohibitively expensive or even impossible to obtain a replacement front element. Please don't take any heed of the arguments that a front filter is unnecessary or expensive. You can always replace it if it gets scratched or damaged and this is its only function in my opinion. It will take all the abuse of repetitive cleaning and you can learn all the lessons you need on a good filter. When purchasing a filter buy the cheapest one you can find. They are all about the same. You would have to work very hard these days to find a bad filter."
Here's the website, btw.
I'm the only guy with this opinion, but my experiments have mostly confirmed this, though I have had a couple of hazy filters. A Sunpak, Hoya, and a Tiffen. These all came in plastic boxes and could have been stored in hot conditions. Some plastics produce gases when they get hot that can chemically etch glass. I suspect that's what occured, though I don't know for sure.
Mostly, I use cheap Zeikos filters (made in Japan) to protect the lens. I've A/B tested them on a tripod with fixed focus and have yet to see any effect on IQ, even when viewed at 100%. I've got probably 15 lenses, and I keep a UV filter on all of them except the fisheye, because you can't put one on it. Never had a problem.
You could try one & if you don't like it, throw it away. You'll be out $3 or so. And in the event you see excessive flare in the viewfinder, unscrew the filter and if the filter is culprit, take the shot & screw it back on when you're done. Not a big deal.
I said the same thing in another thread and some guy got all up in my face about it like I was talking smack about his mother or something. For the record, I'm not telling anyone they have to use this kind of filter or no filter or whatever... I'm just saying this is what I do, and it's worked for me for years now.