It will depend on whether they're dwarf, semi or standard as well as variety.
If no fertilize was used at planting then fertilizing at or just before green up is beneficial. I prefer to use composted poultry manure because of the additional nutrients such as boron, calcium, sulfur, mg and other trace minerals, that apple trees need. 10-10-10 will work when the trees are young, but as they reach 5,6 years you should scale back the nitrogen.
Also, apples do best with a PH of 6.3 to 6.7. Spraying for insects and diseases is very important too.