Not really necessary since the context says it all. Many Swedes have problem when it comes to writing though, because they haven't really picked up the difference between "de" and "dem" since both are pronounced the same way.
Sounds like how native English speakers are constantly getting their/there/they're and your/you're etc wrong.
That kind of reminds me of the zij-hen-hun issue in Dutch; although completely unrelated, many people get it wrong: Native speakers commonly use hun (them, indirect obj.) instead of hen (direct obj., or ind. with preposition) (which isn't even considered wrong by quite a lot of people), or hun instead of zij (they), which is considered wrong though. I don't even get why they get it wrong
De is subject, dem is object. They're used the same as they and them respectively in English.
Can you use dem for things too, not just for people? For example if I am talking about knives (in plural) Is it right to say 'Jag tycker om dem' ?