Yes, I agree with rjpond. If you should use "whom" when it should be "who", you can risk being hypercorrect, and hypercorrectness is repulsive to a lot of people, many of whom will roll their eyes at you or take you less seriously if you should get caught up in hypercorrectness with your grammar. One of the biggest hypercorrections in English is using the subjective pronouns when the objective pronouns should be used or using the subjunctive when the indicative should clearly be used:
between you and I (wrong) = between you and me (correct)
I wondered if / whether he were there (wrong) = I wondered if / whether he was there. (correct)
In the first example, the preposition "between" signals the use of an objective pronoun, but many people have had the "you and I" rule for subjects inculcated into their minds since they were wee children, so they just assume that this is always the case; they fail to understand that the rule only applies when the pronouns are subjects of the clause or phrase rather than objects. In the second instance, people often try to use the past subjunctive "were" whenever they see "if" or "whether" because they think they are supposed to; however, they fail to conceptualize that these subordinating conjunctions don't always take the subjunctive mood. In both instances stated above, they think they're being really correct and formal and, thus, they think they sound smart, but, in reality, they sound really bad. Whenever I hear "whom" and "whomever" used incorrectly, it grates my ear and it can detract from the message the speaker is attempting to convey to me.
As rjpond avers, when in doubt between "who" and "whom", always go with "who" as native speakers consider this to be merely an informal utterance; the opposite, however, is just flat-out ungrammatical.