Saliva is normally and ideally acidic. Why? Well it has to do with the salivary enzymes - the most important being amylase (digestion) and lysozyme (antibacterial). Like all enzymes they have a range of activity (i.e. how fast and efficiently the work), that activity is dependent on several things like like temperature, pH, and others.
Lysozyme for example is most active at pH ranges from 3 - 7, though it can still function as low as 2 or as high as 10 but at limited activity. http://jdr.iadrjournals.org/cgi/reprint/55/6/1137.pdf
as you can see for humans the activity peaks at 6.5
Simlarly salivary amylase works best under 5.6 - 6.9
http://en.wikipedia.org/wiki/Amylase