Both methods are correct, and each has it's advantages. Using a windowed valve cover to adjust flat tappet hydraulic valve trains while the engine is running is an accepted method which goes back WAY before these SBC engines were even invented. In fact, many companies sell oil deflecting rocker arm clips for just that purpose.
Here are two such examples:
http://www.summitracing.com/parts/sum-140355
http://www.spiderautomotive.com/rockerarmclips.html