Although my preference is forced air, we've had baseboard heat since 1990. 24 years with baseboard electric and 7 with hot water. Using some common sense, we've never had any problem.
In my experience, baseboard units are almost always located on exterior walls and under windows. With the normal covers in place, as Irwin said, they don't get hot enough to start a fire as long as there is reasonable room for air movement. You don't want to create a situation that causes serious heat buildup though. Here are a few thoughts:
If a heater cover is broken or missing I'd keep the area completely clear and have the cover repaired or replaced ASAP. (Note: This has never happened to me).
I would never install window treatments that are long enough to touch the heater cover.
The backs of our upholstered chairs and sofas slant a bit outward (backward) from the seats. When placed in front of a heating unit, if the top of the back is 3 or 4 inches from the wall that seems to leave plenty of room for air circulation.
An occasional table, curio cabinet, or bookshelf with legs that raise it above the heating unit isn't a problem, but you might want to leave an inch or two of space behind it, depending on its size.
We have an eight foot long credenza that sits directly on the floor. I would never put something like that in front of a heating unit.
Never pile clothes, papers, trash, &c. on top of a heating unit. Do not store flammable liquids near a heating unit.
Hope that helps.