how to avoid tiny gaps in end joints of hardwood floor install
I'd use wax filler or even colored caulk (sometimes sold as wood mastic) -- a tiny amount. Let a bit of it dry first and compare the color to the wood--it can change substantially as it dries/hardens. Some laminate flooring I bought last summer (made by Classen) came with a wax sealer pre-applied.

For a solid (or even multi-ply) wood, sanding is sometimes an option, but for a beveled (v-groove) edges like yours it will probably look a little odd anyway and it's more labor intensive, but you can reshape the bevels this way. This requires pulling out the pieces though. If you really wanna go this way use a fine grit, 180-220 at least and an orbital sander. Try your technique on a piece of scrap first. You'd have to apply varnish after that and chances are it won't look exactly the same as the factory applied one. So I'd leave this route for when you redo your whole floor.