Mark's right. Aniline is NOTORIOUS for containing impurities, because it's a somewhat-easily-oxidized primary amine, when exposed to air (among other things).
How will it affect your results? I'm not expert, but my 3rd-year-University guess is that the overall concentration of aniline won't be what you think it is. You'll assume you have 1 mL of pure aniline but you'll really only have like, 0.8 mL and 0.2 mL of impurity (not sure of the numbers...because they will vary based on how much your aniline is oxidized!!). Anyways, you'll have to figure out what the result of air oxidation of amines is...and then see if that increases or decreases iodination (electrophilic aromaic substitution?).
I personally think that it'll make the amine-substitutent LESS activating, and DECREASE the overall rate of iodination.