That depends on the frequencies involved and thus the data rate. At 9600 baud probably not much. At 1000000 baud probably enough to matter. Unfortunately I cannot put actual numbers to it, but maybe someone else can. Twisted pairs minimise interference, they reduce the interference given out by one circuit, they minimise the interference picked up by an adjacent circuit and they minimise the interference picked up from the environment. Whether that's relevant in your situation I do not know, but why would you even consider not using twisted pair cable when it is so ubiquitous as both telephone cable and cat 5 cable?
Not much. However, the resistors need to be at the end of the chain. Again, frequency is everything and 9600 baud is probably OK whatever you do, higher frequencies might be a problem. resistors should be at the ends to remove reflections (as in the page you linked to RS485-Modules - ArduinoInfo ).
I would not think an effects would be significant at the speeds I guess you are working at (which you have not stated).