Spar varnish (marine varnish) is waterproof and sunlight-resistant. Naturally, good for use on ship or boat spars and other woodwork where you want a glossy finish. The only note of caution in a high gloss surface is that it can be slippery.

I don't know how long it would take your log rounds to dry out to accept a coat of varnish, but you might want to consider adding some sand or grit to the surface before the varnish dries - simply for safety's sake.

Not intended as stepping stones, but I've got the trunk remains of old spruce cut down to maybe 5 inches above ground level in my fern bed - maybe two or three times as large as your cherry logs. I've slid off them more than once and they're just bare, unsealed wood.

I wouldn't bother varnishing them because you'll have to wait too long for the wood to dry. Just use them, in actually they are free money.

Here is a link that might be useful: Propagating Perennials

Bullthistle...

What's up with the perpetual posting of "propagating perennials"? In what way is a site dedicated to propagating perennials useful to someone with a stepping stone (or log) question? Perhaps I missed something...

Stepping stones usually are used through a garden or lawn area and other than vegetables what do you have in gardens?

Here is a link that might be useful: Propagating Perennials

Apart from them becoming slippery even if stones are used instead of wood rounds you still have the problem of them being dribbles in the scene that produce a visual distraction from the whole. When looking from afar your eye is captured by these dots running through. When walking on them you have to look at them to see where to step, instead of taking in the garden around you.

Where individual stones are useful is in mixed borders, as islands among the plants where feet can be placed instead of trodding on the soil (or plants).