Oh wow, so many questions in response to that... It is a really broad question and the answers have loads of "it depends"!
First of all, assuming you have integrated your Marketo instance with a CRM, I would try to always use smart list filters to select your audience based on the data Marketo already has rather than using list uploads. You could also (again, depending on how your CRM is integrated) assign people to the campaign from your CRM. If you have just migrated from a traditional email tool it is very hard to get your users to trust the database filters you have available, but it is well worth investing in building that trust.
List uploads can be tricky and I have seen plenty of database issues created by the practice. A few things can help you keep this under control:
- Ensure only people who understand the database structure get the permission to upload lists.
- Having a template with the fields to include (marking mandatory vs optional fields clearly) will definitely help.
- Identify fields where existing data may never be overwritten from a list upload and set the block field updates accordingly.
There is a lot that can be / should be configured in your instance's data management processes for new records which will define what you should or should not include in a list upload.
I hope this at least gives you an initial sense of how to go about it in your particular circumstance.