Singular and plural are (more or less) synonymous.
Regarding your main question, I would try to make sure that there was no subtle difference in user intent that you might have missed.
To use the example that you give someone looking for "mountain bike training" might be more interested in formal training than someone looking for "mountain bike workshops" while someone seeking "mountain bike instruction" might only want to attend a single session . It might even be worth testing some theories out using PPC.
You could also consider branching off slightly from the keyword tool's suggestions. Again using the example you give you could create a page about mountain bike instructors - possibly optimising it to the term "mountain bike instruction", using it to write relevant content but with a significantly different angle to your other pages and also as a way to build relationships with the community of mountain bike instructors.
