Best practices for using Content Types

So, It's been almost a year since my last activity with content types. Thought I'd give it another try. :)

Still having a folder with business documents, like invoices, letters, proposals, etc. Some are mine (sales invoices, proposals), most are from outside sources; invoices and cash receipts, letters, etc.

As to your tips, I want to keep it simple and don't use a lot of columns. Just the ones that I need to automate stuff and to sort and filter. I want to start with the invoices and receipts, as those are the majority of the files. 

First question: I have invoices and receipts. They are different content types or not? They share a lot of columns, as they are both about financial transactions, have a date (invoice date... and receipt date) and maybe more. But an invoice is about a request to pay, while a receipt is an prove of payment. (most of my receipt are from drinking coffee with customers :-) ). So how do I create relevant content types and don't double up on columns or get in any other troubles later?

Second question: I thought it would be usefull to create calculated columns for Fiscal Year and Quarter. This seemed logical to group documents in a library or search or filter. So in my first attempt I created an Invoice Content Type and a "Invoice Date" coulumn. Then I added two calculated columns, "Fiscal year" and "Quarter". But then I started thinking. More Content Types will want to have a column Fiscal Year and Quarter. So creating them and using "Invoice Date" as the source to calculate, seems unlogical when I want to add them to a "Proposal" content type later... or whatever Content Type...

So how do I solve this. Do i create a general "Document Date" column... and base my FY and Quarter on this? Or do I use the "Date Created" column of the Document CT (which I guess inhearits it from Item/System)?