Do I really need spare roof tiles before installation day?

YES.

A good installer will advise you of this and it's a great reason for them to do a site visit and positively identify what kind of tile you have. If you don't have spares left over from the original build they should be able to point you in the direction of a salvage yard or tile supplier in your area.

They may be glazed terracotta (known in the solar industry as TERROR cotta) or concrete tiles but either way there are some which are quite robust and many that are horribly brittle. 

I have had first hand experience with tiles that seem quite good. They handle traffic quite well during the framing and wiring stage of installation but as soon as we start marching the panels out, an extra 20kg per foot fall begins to break them at an alarming rate. 

We often find cracked tiles that haven't been noticed before in any case, so these must be swapped out from under the array and put out over the eaves at very least.

Drilling a tile for a roof penetration can also be a hit and miss affair, so once again having a spare is essential. 

So please make sure you have tiles available on install day.

Tiles are most often ground out with a diamond blade on an angle grinder.

So then they will sit flush with the the bracket in place.

Sometimes temporary repairs need to be effected.

And measurements taken.

Wet diamond drilling is safest

And the tile cleaned if there is to be a flashing glued to it.