Hi there @wasfrans
As said before, there's no need to use all of the RAM installed in your computer unless it is necessary.
By necessary, I mean if your scene has a lot of geometry, has a lot of materials applied to objects, textures are very memory-expensive, etc.
And as @Tom said, "If RAM maxes out, then you can run into crashes and slowdowns" or any other unexpected behavior.
It is always best to have your scene project organized and, most importantly, optimized; I mean, using rendering instances, low and high poly objects (depending on the distance from the camera), keeping materials setup as simple as possible, etc.
I hope this helps.