Is it required to have vents on your valve covers?
Legally, no, reality, yes.
There is ALWAYS some pressure seepage past your rings, it has to be vented somewhere, or you pressurize the engine, and that WILL blow a gasket someplace, my guess would be a valve cover or an intake manifold corner. possibly an oil pan corner too.
If you have a LOT of oil blowout, you may well have excess blowby which you will find impossible to vent away.
Hopefully your new valve covers are baffled to help catch the oil.
Start with a compression test (dry followed by wetted with a bit of oil), a leakdown test would be even more helpful, looking for poor ring seal.
I have ALWAYS run a PCV, and I feel most people's oil blowby problems that they patch up with 4 breathers or puke tanks, could be cured by the engine sucking it's own pressure out of itself (which is exactly what a PCV does, it lets intake vacuume pull the blowby fumes out of the engine).
Not every item that sprang from an "improve emissions" standpoint back in the late 1960's, was a bad idea, note "open-chamber cylinder heads".
Some people insist on running a vacuume pump to reduce crankcase pressure, so do I I guess, I just use the engine itself, not an extra belt-driven accessory and hoses all over.
There is one hell of a "fog" of oil mist droplets in an engine running at high rpm numbers, oil being sprayed and slung EVERYWHERE, Some of it WILL go up the breathers, the more pressure you have (from excess blowby), the more will get pushed up breathers
I run a PCV on the right side, an un-baffled breather ("mushroom cap" looking, twist-in Mr Gasket chrome special (filled with a fibre material) where I fill with oil) in the left side. My M/T valve covers aren't baffled, but, apparently, I don't have anything positioned right over a rocker squirter either.
I get a little oil that drips on the valve cover, after a year or so of use. Wash out and let the fibre stuff dry, and it "catches" just fine, for about a year.
I consider that a "no problem" level.