Should wine be filtered and how much?

The short answer is that this is a situational issue in winemaking and there is no correct answer.

That being said, virtually all commercial wineries filter their wine to one degree or another, either for stylistic or sanitary/safety reasons.

Filtration is relatively new in winemaking.  To give you an idea of the range of filtration options, you can filter your wine all the way from a 10 micron filter (think bug catcher...big particles only) all the way to a .05 micron "sterile" filtration that will take out all remaining yeast and bacteria.

There are two types of filters:

This is the pad type.  It uses pressure to force the wine through a series of pads (number dependent on the system and the quantity of wine being processed)

This is our state-of-the-art cartridge system, which we think is the coolest thing in our winery.  It uses a cartridge which looks like is pushed through and into the bottling line.

As home winemaker, here is roughly what you are looking at with various filtration levels:

10 micron:  This will clean up your wine.  It is likely that you will have some sediment if you plan to let your wine sit for a few years, but not as dramatic as unfiltered wine.  We have tested wine at this filtration level and can't tell much difference in taste.  All filtration "beats up" wine significantly, and your wine will need time to "put itself back together" before drinking.  (What is happening here and with bottle shock will be explained in a subsequent post)

1 micron:  This in known as a "wine polish."  This is a pretty standard micron level for commercial operations.  Roughly, it will remove particles 1/10 the size of a 10 micron filter.

There is a noticeable change in the wine at this level of filtration.  Calling it a "wine polish" is the best way to describe it, the wine is BEAUTIFUL after filtration.  It will clear it up substantially and the wine will literally sparkle in the light.   Esthetics are important and there is nothing more stunning that a beautiful glass of sparkling wine.   We also think that wine that has been through a polish-level filtration has better mouthfeel.

The tradeoff is some loss of flavor.  I think it is noticeable, and we don't filter our personal reserve at this level.  However, many of our wines (all whites, rose, and our lighter-drinking reds) are all filtered at the 1 micron level.  Most people could only tell the difference if they were drinking the wines (pre and post) side by side and that is unlikely to happen.  (except at our house:)

It is also possible that filtering at a 10 micro level could allow spoilage organisms to remain in your wine.  This should be considered if you plan on a long-term aging.  It shouldn't be a problem if you have done everything correctly, but it  can happen to even the best winemakers.  The only way to completely prevent this is a sterile filtration.    .5 (Sterile) filtration is used when you absolutely need to remove "everything" from the wine.  

We make a lot of wine for our own personal use that we never intend to sell in our winery.  We don't filter any of it, we just rely on the age-old technique of multiple rackings.  If we do filter, we never use anything more than a 10 micron, and that is mostly just so we can use our small wine pump (attached to the filter) to more easily move the wine around.  In our opinion, nothing beats unfiltered wine right from the barrel.