Never use the -a option together with -f, that's just asking for trouble. In fact, never use -a, period.
If you want brain-dead simple upgrades, then stick to binary packages.
Then, and only then, should you consider if things need to be updated. And then you should do small batches of updates, in the order listed in UPDATING.
There's nothing wrong with using # portmaster -a. All it does is go over the list of installed packages after which it'll build & (re)install the ones which have a new version. The poster has a point that using -af together can cause some side effects (force compilation of all upgradeable ports and all their dependencies) but other than having to wait a (very?) long time for your builds to finish it won't cause any trouble or something.
I also think context might matter with the quote above. Because I can come up with situations where its use wouldn't be the best of ideas. But generally speaking... no worries.
Make sure you read /usr/ports/UPDATING. Sometimes ports need to be rebuild in a specific order and portmaster(8) won't do that automatically. Which is the main reason why portmaster -a would be bad to do.