I wouldn't say its necessary for your first knife, but its really your call. If you plan on owning multiple down the road, it would be a great idea to invest in the tool. I would also highly suggest inventing in a GOOD set of Precision drivers Hex / Torx. Most companies I've seen use Wiha.
Like others have mentioned, I just use a penny for the main pivot. The only time I wish I had the RHK tool was when I used some blue loctite on a pivot and went to change the scale out after. At the time, I didn't have the RHK tool, so I just used (2) pennies and a vise. It took much longer to eventually get the pivot loose, but it was doable.
Needless to say, after that experience I went to buy the RHK tool to avoid that mistake in the future and haven't used the tool since.