20 psi is quite low. The usual autoclave cure for sandwich is 35–50 psi. If your core is very light (e.g., 2 lb per cu ft Nomex) it might need a lower pressure. If it's not precured the state of the backing (bag side) skin might be a bit questionable with just 20 psi. On the other hand you'll get less edge crushing with 20 psi. If aesthetics aren't a major concern a bit of edge crushing does no harm, but nice sharp ramp edges do look 'professional,' so some people like them to please customers that little bit extra.