No printer can print into "thin air"; each layer must be built onto something else. If a model has no overhangs at all, it doesn't require support. If it has a 90-degree overhang (that's not "bridging" to somewhere else on the model), it's probably going to need support no matter what. Angles in between may or may not need support, depending on the angle of the overhang, printer parameters, filament parameters, how perfect you want the print to be, etc.
I think slic3r has support generation features where you can set support density, threshold angle, etc.
Support removal from PLA prints is generally more difficult than support removal from ABS prints. Some printers with proprietary firmware/software (like the Up/Afinia and Zortrax) generate support that's more easily removable than that usually generated by current open source tools.
julia