I agree about getting it level. Be super fussy on that. I made my own kitchen this year but employed professionals to fit quartz worktops. 6 metre long, extra wide worktop with sink cut out, and 6 metre by 1.4m island with three rebated flush appliance cut outs.
The guys doing the templating and slab fitting said that in 8 out of 10 cases, the worktops are not level, in most cases nowhere near level, and they end up dealing with it with silicone and shims quite often as the kitchen fitters are unhelpful. If they do not install level, then the upstands look terrible.
Also in Mychal case, because of the size, 3m slabs of quartz had to be joined. (They did a fantastic job - near invisible). If you are joining work surfaces then it is essential to have the mating areas dead level and well supported.
Be super careful getting your oven / hob dead on level in all directions. Otherwise fat runs to a low point in pans. Very annoying if you are a cook.