Thanks for clarification, this makes sense of course. I think it depends how many projects are running together. If there are a lot projects (= a lot more search queries), then it's possible that search engines ban the ip when no proxies are used.
In proxy settings there are multiple options: searching, posting, everything. I'm not sure what "posting" means. Is it just used to post the form, or does "posting" mean everything beside searching? I've also the problem that bandwith on proxies is quiet expensive, not the proxies themself and "everything beside searching" would consume a lot bandwith.
I doubt the number of projects will make a difference. I gather the program works on x threads (user set, eg 50), and divides them amongst projects. So with only one project, whilst scraping there would be 50 simultaneous connections, with 5 projects, the same amongst.
In fact, as all 5 projects are unlikely to be in harvest mode at the same time, it would seem more projects would mean less simultaneous connections to a search engine.