Vitally Important!
If you must use a physical delimiter use a comma. Otherwise things get messy once you have a decimal point in there.
When I did big engineering sums back in the day (limited time on the company's IBM main-frame and Dog forbid you sought permission to apply for authority to book time on the UMIST brain...) and had to write it all down manually, a space every three was the norm (so that even the French could understand it, since they insist on using a comma as a decimal point).
Though I much prefer standard mathmatical notation (10^7). You don't even have to count the number of zeros or give it a name.
I have often wondered why we have to work numbers arse about face?
Reading from the right:
Units, Tens, Hundreds, Thousands, Ten Thousands, Hundred Thousands, Millions (modern uasage), Ten Millions, Hundred Millions, etc.
I can only surmise that since our numbers are derived from Arabic numerals - and they read from right to left.