MongoDB relationships for objects

Please excuse my english, I'm still trying to master it.

I've started to learn MongoDB (coming from a C# background) and I like the idea of what is MongoDB. I have some issues with examples on the internet.

Take the popular blog post / comments example. Post has none or many Comments associated with it. I create Post object, add a few Comment objects to the IList in Post. Thats fine.

Do I add that to just a "Posts" Collection in MonoDB or should I have two collections - one is blog.posts and blog.posts.comments?

I have a fair complicated object model, easiest way to think of it is as a Banking System - ours is mining. I tried to highlight tables with square brackets.

[Users] have one or many [Accounts] that have one or many [Transactions] which has one and only one [Type]. [Transactions] can have one or more [Tag] assigned to the transaction. [Users] create their own [Tags] unique to that user account and we sometimes need to offer reporting by those tags (Eg. for May, tag drilling-expense was $123456.78).

For indexing, I would have thought seperating them would be good but I'm worried it is bad practice this thinking from old RBDMS days.

In a way, its like the blog example. I'm not sure if I should have 1 [Account] Collection and persist all information there, or have an intermediate step that splits it up to seperate collections.

The other related query is, when you persist back and forth, do you usually get back everything associated with that record - even if not required or do you limit?