Watercolours should always be framed and glazed, and with the best none-reflective UV resistant glass you can afford. But oil paintings are a different story. Unless you are titled and regularly open your home to the public there is no need to frame your oil painting under glass. If it is painted on canvas, panel, or board an oil painting is best displayed framed but never glazed.
Yes, all of the important museums and galleries exhibit oil paintings behind glass but this is primarily for protection against vandalism. Using anti-reflection Museum Glass® will block up to 99% of harmful UV rays but, believe me, concern about the lighting rig in the Louvre is not why the Mona Lisa can only be seen today behind glass. These days prestigious galleries have more to worry about from lunatics than they do from light or atmospheric pollutants.
For centuries artists have been finishing oil paintings by applying a final layer of varnish, not only to produce an even sheen and saturate the colours but also to provide protection to the paint layers. A professionally applied layer of varnish is usually all an oil painting needs to protect it. In fact putting an oil painting under glass can frequently invite trouble by trapping atmospheric moisture and damp from walls.
If we were lucky enough to be back in Arles in 1888 right now and walking past a window in the Yellow House, I have no doubt that Vincent would invite us inside to look at his Sunflowers rather than have us peer at it through a pain of glass.