is 5% open wounds enough for ubers?

Well, the lifetap you could get from Exile. Or a Wand. Or ...

OW itself isn't there to do any real damage. It is solely there to stop the ubers' regen (because PMH doesn't work on them).

Plus you can't "double-OW" anyways so any OW that gives "enough uptime" is enough.

To add some math to that:
OW lasts for 8 seconds.
Using Smite, you hit 100% of the time so "no application due to missing" can be ignored.
As a result, when you look at the actual odds of re-applying open wounds before it runs out, you get the following (assuming 6 frame Smite):
Based on that (as well as personal experience) there will be gaps at 5% during which ubers will manage to get a bit of regen in but it is still sufficient to get them killed decently well.

Personally, I keep my smiters at 10% OW (thanks to Gore Rider) which is more than enough even if you run a setup that has you dropping Fanaticism (and hence down to 7 frame smites) against meph.