Would it be completely necessary? How old are we talking here?
Better safe than sorry. I looked into SDL's rendering method and it cautioned that the antialiasing greatly slows it down and it shouldn't be used for text which will need to update often and so forth; I'm also not sure how efficient the game's rendering pipeline is set up.
It works perfectly smoothly on my system, but again, better safe than sorry; if there's a config option to disable it, anyone who did have poor performance (or some other reason to not want font antialiasing on) could change it easily and everyone else could simply ignore the option.
As for exactly how old of a system might notice degraded performance... I imagine pretty old, probably, but it remains to be seen.