I've been on some of these flights (generally from South America) and was never affected by this program. It's really hard to predict how many seats will have to be left empty (if any). There are several factors to consider: cargo loads, temps at take off, weather en-route, etc. Cargo loads tend too be lighter on Mondays but this is not without exceptions.
You can get an idea of how things are going by checking how many non revs make it out during July...
I say go for it.