There is no "should" in how a personality type should act. I wouldn't subscribe to that belief. Yes INFPs will hold grudges. It depends on how attached they get at who or what they hold the grudge. You can't help that either. (This wasn't an attack, I was just using this post to support my point)
Yes, to me grudges are a very INFP thing, when I look at the theory of the type and the Fi/Si tertiary temptation. The grudges would be held against someone/something that the Fi user had become very attached to (which is par for the course with Fi).
I think grudges are an Fi/Fe thing, but I'm not sure how an Fe grudge would work. I suppose if someone harmed the group, then Fe, with its concern for the group, might direct anger toward the person who harmed the group. However, if the person who harmed the group was also part of the group somehow (likely), then that would create a problematic situation where the person the Fe user would hold the grudge against would receive special consideration as a group member. (Is that correct?)
[quote
This is mainly because an INFP's relief function is Si, as pointed out earlier in the thread. Si just naturally happens for them. They will remember and take to heart past offenses if Si takes it's toll. If they stay in this Fi Si mode and if they don't gather new information through Ne, the grudge will probably stick within them.[/QUOTE]
I have very strong Ne, but my Ne directs itself toward ideas and information, not people. So, Ne never really gets me out of the Fi/Si loop because it doesn't operate on the same plane as those functions. Instead, it seeks for non-people-oriented solutions to people problems sometimes.