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Best course of action is to have more than what's required.
 
Basics are:
I-134
most recent year tax return transcript (2017 until april 15, then 2018)
3-6 months of pay stubs or employer letter
 
if your income is close bring quarterly bank statements for a year if assets will help
 
optional is the other 2 years of tax returns.
 
Best option, to cover all bases is
3 years of tax return transcripts
1 year of bank statements
6 months of pay stubs
and employer letter
 

Depends on the embassy/ consulate what they want. Consulate in Frankfurt/Germany only asks for the last tax transcript and nothing else of what is listed there. 

It is all explained right here:  https://www.uscis.gov/sites/default/files/files/form/i-134instr.pdf

Yes most filers do submit their tax returns to include tax transcripts even if they are NOT self employed. If your income is above the 100% threshold there is no need for the banking letter, but letter from your employer yes include that. I did send my tax returns and transcripts to my fiancee in case she will be asked for them. It's better to be prepared than not. I might add that Morocco is a very tough embassy so you will need every edge.