I usually get my MDF at Gamma, Hornbach or Praxis. most carry standard sizes of 122x60 cm in a variety of thicknesses.
I had one of those (3mm btw) cut into 4 pieces of 30x60cm at Praxis when I bought it, and the cuts were spot on in my case. the left-over strip at the end was about 1cm wide, so the loss from 4 cuts
was considerable. Had I had them cut the board lengthwise, I'm sure the parts would not have been 30 cm tall...
It also depends heavily on who does the cutting; some are better (and know their machines better) than others and I suppose this varies even within the same store.
If exact dimensions are really important to you, you could either do it yourself (but you'd need to invest in the right equipment), or maybe use one of those lasercutting services that seem to be sprouting up everywhere. I've been eying this one to use for a terrain project I'm planning:
https://www.snijlab.nl/nlRelatively expensive, but if it's precision you seek, that's a sure way to get it.