...that is anti-aliasing (smoothing), you can turn that off in the illustrator (and acrobat) preferences, it a screen render anomaly, and isn't visible on high end vector output...
...because you render a tif to pixels you will however notice it, because it has to anti-alias, smooth out the text...
...as a general rule you would want to keep it vector wherever possible, rather than raster pixels, if you must render text to pixels for high end output, you can get better results if rendering to 600 - 800 dpi...
...unless no trapping spreads applied to artwork by prepress and the printer mis-registers inks, your not likely to notice...
...like a lot of these kind of things, the average person would not know to look, but once you notice these things they tend to stick out more and annoy...
Andrew