The thing to be remembered here is that prior to the introduction of Statutory registers in 1855 in Scotland, these are church parish registers and not marriages (or births).
Generally they are considered as "proof" of marriage, since no direct marriage register exists in most cases, but yes these are usually records of the proclamation of the banns, rather than the date of the actual marriage (except where as mentioned you have a further addition of "married on ...). The same often goes for births, where in many cases what you will find is the record of the child's baptism, with the actual birth date sometimes but not always included.
Ultimately what you add to your tree is a matter of personal choice. Some put the date given as the birth or marriage since they have no other date they can use, others scrupulously list the baptism or banns and simply give the actual birth or marriage as "about."
One other detail - once the banns were announced, if the couple subsequently DIDN'T marry, that would often appear in the kirk session records, so if you have access to those and no mention appears, you can be pretty confident of the marriage taking place.
