Precautions
Galvanic reaction
Do not allow spacers to touch dissimilar metal (spacers are stainless steel). Where necessary, spacers may be bent or cut. With the exception of hooks (which are generally stainless steel, and so not a problem), spacers will not normally touch other metal elements.
Nail depth
Properly used, spacers increase the margin for error when setting nail depth. But the normal precaution applies: Do not leave nail heads high. Spacers are an aid to avoid over-driving nails, not permission to under-drive them.
Joint space
Installers are accustomed to leaving a small space between horizontally adjacent slates. This helps each slate to hang freely; it gives debris a clear path, and it adds textural interest. Maintain this practice, and be sure that the space is at least 3 mm (~ 5/32").