On the same thread as commenter @Barmar : unused RAM is wasted RAM.

There is no significant impact to general performance between a device with most of its RAM unused, and some of its RAM unused. In this regard, what matters is that you are not approaching or exceeding all of your RAM used. As long as the systems are not doing so, it is fine.

This does not necessarily give you an answer, though. You need to understand the systems it will be deployed on, and the processes you expect to them to be doing, to inform your decision.

So, if the systems running the memory-hungry script is solely used for that purpose, then there is no reason to keep excess memory free. While running the script, there will be no other processes using up memory, and so you essentially can know empirically how much RAM you can get away with.

However, if the systems are not only for data processing, you need to factor in the potential for processes outside of your own, and subtract that away from you allowance. For example, if you expect to be running this on someone's personal laptop, you have to account for the possibility that that person may open up chrome, play a game, have an application auto-update, or all of those at the same time. Ignoring this will not only make the device unusable for a period, but likely cause overflow errors which may halt or even effect the validity of your data processing. In this case, it may be beneficial to be conservative with your usage. One might see how much RAM the processes they expect to run concurrently might take up, and subtract that from their allowance.

TLDR:

Unused RAM is wasted RAM. If it is the only thing being ran on the device, use as much RAM as you have. If not, conserve RAM for expected tasks, based on the actual usage of those tasks.