There's a significant terminology issue at work here. When you say "umbrella", what EXACTLY do you mean?
I ask because I've seen positive seals erroneously refered to as "umbrella". A "positive" seal has steel rings around it that "secure" it to the top of the guide. An "umbrella" just slides over the valve stem and "rides" up and down with the valve. Earlier versions of Ford V8s made extensive use of unmbrella seals. SBC only had them on the exhaust side, circa 1979-later (usually a hard plastic material). The SBC also used the positive seal on the intake side during that era. Today, virtually ALL engines use a "positive" seal.
All that said, the "O" ring at the top is intended to prevent oil from runnning through the retainer, down the stem into the guide. While they MAY be extraneous, GM DID use them in conjunction with the positive seals and umbrellas. In short, using them ("O" rings) couldn't hurt!
Jim