Would a number, say,
be written with commas:
Five million, six hundred twenty nine thousand, two hundred ninety six
or without commas:
Five million six hundred twenty nine thousand two hundred ninety six
It's a style guide thing in my op, but if you're writing out numerals that large in a block paragraph, I would recommend using the commas for readability purposes - a string of text numerals is hard to parse - and it also sounds better (I'm relatively sure most people insert pauses between logical digit groupings).
I'll admit to influence being ex APS (Australian Public Service), but we do tend to do a lot of writing and our style guidelines have been hammered out specifically with a view to making sure that the relevant information gets to the eyes of the target reader, in a form with the lowest chance of it being mangled.
Paraphrasing our guide (specific reference below) and personal experience, three other suggestions:
I thought it was a bit silly when first having to internalise the style guide, but after a while it starts to make sense, for example in a straight side-by-side:
Of course, your internal guides (or the prevailing style in your area, I have a funny feeling AmE might drop the internal ands for instance) will have their own suggestions, and consistency is king with something like this.
Source: Old habits from a stint in the Australian Public Service as a policy officer, and the Style Manual (6th ed), pg 176. (ISBN 978-0-7016-3648-7)
I think this is a question of readability. From my own experience the use of commas and the breaking up of the numbers into groups of three is optional, however in some ways has become a convention. When converting this into text, the same would apply. In most cases, spelling the number with the same commas as used when in numerical form facilitates readability. The only case where this may become confusing is if you were listing large numbers in a sentence, in which case leave them out.
I think the general consensus with this is whatever makes it easiest and clearest for the reader to understand.
"Two hundred and ninety-six" or "two hundred and fifty thousand" are both wrong. In the first example, the "and" denotes a decimal point. Converted properly into digits it would be: 200.96. In the second example, the "and" serves as a plus symbol. Converted properly into digits it would be: 200 + 50,000. The word "and" has no place in a properly written or spoken whole number. The commas should be placed like they would be when written out as digits. Ex: One hundred eleven million, one hundred eleven thousand, one hundred eleven would be 111,111,111.