Depends on your castings.
Sand castings as the name implies are done in...well sand. The casting will usually have some of this stuff left that is abrasive to tools and machine. Tumbling, wheel-a-brating and cleaning will reduce it, but still some left.
Other casting methods do not have this problem but can be more expensive in short runs.
The normal casting problems are materials and dimensions.
Material - Hard spots, breaking thru a tuff skin plague tooling and tolerances, Depending on your foundry these probelms can be greatly reduced by the melt and cooling methods used.
Dimensioning - Bad castings are a pain to machine, from trying to figure out datum lines and have casting cleanup at critical points can be a job in itself.
We had one job that was simple, just bore a 1.750 +0/-.003 dia hole, 4 inchs long in a bronze casting, casting had a pilot hole of 1-1/2". Could have been done in one pass. Bore just had to be centered and cleanup. However, the customer supplied castings came with a pilot hole of 1-1/2" and were never on center or parrallel to the OD so each casting had to be indicated top to bottom and fixture realigned. A five minute job became five minutes of machine time and 10-15 minutes setup time.
Anyway a good foundry should make your parts easier to machine and cheaper in the long run.
