Guidelines for When to Acknowledge Research Assistants
I think it's best to err in the direction of being generous with attribution.  Firstly because it's hard to be accurate about how important someone was (which both applies to you potentially underestimating your contribution, or the RA overestimating their contribution) which can lead to hard feelings.  And second because it's just a few words and the power to make someone feel good about their contribution outweighs the cost of those words.