I have two nice red oak bokkens, and two nice wooden ash wasters (one hand sword). They've been used with contact, and against wooden post. Both are varnished (or, in the case of the wasters, slathered in teak oil).
Two have held up well; two have splintered. Those which splintered have, in their grain, successive "V" shapes or chevrons on the edge. The bits that splinter off are chevrons or sections of chevrons. Those which have held up well have the chevrons on the flat (very nice to see), but, on the edge (as much as possible) only the long parallel lines of the grain, sandwich-like in alternating tones. I don't know the technical terms to describe woodgrain but am aware that there are such terms
Obvious, really: chose a wooden sword with an eye to the grain-- parallel grain on the striking edge. I would also have thought that sword makers would have paid attention to this detail. But this makes ordering a wooden sword unseen trickier.
[edited to add a detail]