Take a look at this question:
How can I carry a tripod on my bike?
The question itself is about how to haul [item x] around. The answers get into panniers, DIY stuff, and safety issues, all of which we have tags for. Should we be tagging for as much of the stuff in the answers as we can? (There's a limit of 5 tags per question.)
Bonus points for setting up your answer so this meta question has the same problem. :)
I think the better answer for this specific kind of problem is to come up with a tag we can agree on for "carrying stuff on your bike". "cargo", "luggage" and "utility-cycling" are existing tags that sorta cover this area... "carrying-stuff"?
If you're trying to figure out how to carry item X, you're not going to look for panniers, racks, baskets, rack-bags, backpacks, messenger bags, totes, bungees, ties, straps, long-tails, trailers and everything else you could possibly use to carry something on/with a bike. In fact, odds are good that the things you can think of weren't quite right and you need one of the other things that you didn't know about or forgot about...
In other words: I vote for tagging on question and the underlying problem asked about in the question, not tagging on the specific solutions in the answers.
You can approach a two prong solution too, where you borrow from @frei 's answer, but if you're only at 3/5 tags, it would probably be helpful still to add supplementary tags. I.E. if a question uses 2 tags, has a 'cargo' tag added; then the two top voted answers are a pannier and a messenger bag, go add both of those tags to give people interested in those tags situational awareness as to their 'interested tag.'
For instance, on cooking.se there is this question that is decidedly not vegan; however there is an answer that is a) very good and workable and b) vegan. As long as the tags are hierarchical, this could add value to the tags in and of themselves. Personally, tags are my primary way of filtering the .SE sites.
