Not all cast material is porous. At least not to the point where the pores will cause weakening of the part. The greater concern is what the metallurgy consists of, what sort of heat treat (if any), and casting flaws, such as porosity, shrinkage/cracks, alloy segregation, etc. I doubt that at the price of cast spindles nowadays, any quality control is being done. At a minimum, pretty much all new cast parts should be magnetic particle inspected for surface defects prior to use.
As far as welded spindles go, unless you know that the welding was done correctly, taking the parent metal composition into account, you may also be taking a chance. I would get the spindles magnetic particle tested, primarily in the weld and heat affected zones, and the clean up any irregularities on the weld with a die grinder. It wouldn't hurt to run a 3mm tool steel ball along the edges of the weld until you can no longer see a line with a 10x loupe.