You should lap any time you are uncertain if it is flat. If it's already flat, then the lapping process only takes a few seconds and removes insignificant amounts of material. If it isn't flat, then you'll be glad you did it.

When I first started, conventional wisdom was that new hones didn't need lapping. I thrashed around for nearly a month before I threw conventional wisdom out the window and lapped it, resulting in an immediate and significant improvement to my honing, and I was finally able to produce a shave-ready edge.

FWIW, this issue of getting lapping grit stuck in the hone only seems to happen the first time you lap it. It hasn't happened to me since then. I suspect that the hones have a sealant on them, and the grit is sticking to it that first time you lap it.