No you don't have to use RND when checking page rank, it's just there as an added option. If you're using shared proxies it might benefit setting a few seconds delay in between PR check. If you select the RND option it will change to minutes instead of seconds so make sure you use a set delay in seconds.
OK, thanks. Then I made a mistake when I sett it to OFF.
And I am using the proxies that I harvested in SB.
I have one follow up question. When I check the PR and I see some sites that have PR6. When I then visit this site in my default browser I see that this site has no PR. The same is with PR5 and PR4.
Hmm that's odd. That's a question you would have to ask the developer herself. I'm not sure how or where Scrapebox pulls it's data to calculate page rank so I really can't answer that.