They'll even write '90's to shorten 1990's. That will work fine as '90s or 1990s.
You don't need to write their's. Theirs is already possessive (like yours). On the other hand, people write ten's of thousands. It's not possessive, so you don't need the apostrophe. It's tens of thousands.
In most cases, the rule of thumb is to ask yourself if the word is possessive (Tiger Woods' golf clubs, the Green Bay Packers' quarterback, Green Bay's fullback, summer's end). If it's not possessive, you probably don't need an apostrophe.
One more thing: If you said that James owns a green car, you might say that James's car is green. It's better to say that James' car is green. Editors and school teachers will thank you.
More EDITOR@WORK blog entries
Blog entries from The Auto Racing Journal
(a book of great stories about the Intimidator)
(the book of great NASCAR stories)
More blog entries by Tom Gillispie
Anecdotes by Tom Gillispie