hi coach,
Whatever works best for each screw hole.
Once you plan out where to put your shelf support screws, what will each screw go into ? Brick, cement, grout, or something else? Try out all the options; none of them requires more than a drilled hole.
Personally I wouldn't feel comfortable using Tapcons as a primary or first priority approach in a cement block and especially not in the hollow part. Too thin for my liking, considering that the tapcon threads are going to stress the hole's surface which was made of a material designed only to be part of a continous surface of a hollow brick. It's not as bad as putting a tapcon into Durock (or any other CBU backerboard), it's just that I wouldn't use tapcons as a "first line of attack/defense."
The stress a shelf puts on the wall is shear. A wall shelf will have several fasteners; stress is spread out over all of them.
I would use some toggle bolts and some lead anchors. Both systems.
Why? My approach is to diversify instead of concentrate on one method alone, as that helps ensure that the strengths of each type compound together and the possible weakness of a type do not become a weakness throughout the whole system.
That is the idea for using different fasteners.
DAvid