When creating a mod, aside opening skyrim.esm is it better to check update.esm or rather not? As far as I understood if it is checked you need to have the latest skyrim version to run the mod, if it is not any version would do. But what are the advantages/disadvantages of checking it or not?

Generally, you should always use Update.esm as a master, lest you potentially undue fixes made in the offiicial patches. Even if you are going to be changing a record that Update.esm touches, and thus overwriting it anyway, you are still better off working from a fixed foundation. Arthmoor or one of the other Senior Modders could probably give a more indepth explanation, but that's it in a nutshell.

Interesting, so it is corect to say that the wider the scope of the mod the higher the need for update.esm to be active, while instead if I modify a tiny little thing (say the cost of a single spell) chances that I screw up something related to the update are minimal? Also, but if I check it, do I force people to run the latest version to use my mod?

I was working on a mod, and it wouldn't load, because of problems with Update.esm.

Since losing that progress, I've avoided update.esm like the plague...