Is it acceptable to install new cabinets parallel with the floor rather than plumb and level?
Step one of redoing the slab is pulling those cabinets back out. IMO, only you can define acceptable. 1-2y, eh? So, like anywhere from 5 years to "temporary-permanant"...? 

Just do it right or the doors won't stay closed and all your carrots will roll off. It'd be easier and better to shim the piecemeal cabinets, instead of the entire counter top along each and every one of its contact points all at once.

You have to shim one or the other (sometimes both), why not do it from the get-go? Resign yourself to pulling them out when the time comes and life will be a lot easier. Having "plans" is no excuse for schlock in the meantime.