When should I use an existing field?
Consider the benefits and drawbacks of reusing fields:


Performance: There's a slight performance gain from reusing fields. However, for most sites with moderate numbers of fields, it's a negligible difference.
Maintainability: In theory, if you re-use the same field for the same purpose on multiple content types, then if you decide to change its functionality later, you only have to change it in one place. Similarly, if you want to theme it the same way across entities, you only have to use a single selector. In practice, this rarely happens, and more often the converse is true- you've used the same field in multiple places, and then realize that they actually need to behave differently.
Setup: Obviously, it takes less time to re-add an existing field than to add a new one.
Portability: If you export fields with Features, it's best to not reuse them between entities unless absolutely necessary. Technically, there's nothing wrong with reusing fields (since you can export field bases and field instances separately), but in practice it tends to just lead to confusion and circular dependencies. If you choose not to reuse fields, then you have the luxury of being able to prefix them with the content type / bundle they are attached to, e.g. "field_article_image", "field_page_taxonomy", etc..., which eases portability and maintenance.


In case it's not clear from my tone there, my preference is to never reuse fields unless it's totally, completely, 100% clear that they really are, and forever will be, used for the same purpose.