I have posted many many many comments for quite a few campaigns using scrapebox, and this is what I experience.
95% of the things I do with scrapebox, I do using harvested public proxies. This includes PR analysis, scraping, link checking, index checking, blog analysis, and A LOT of posting.
It is important you keep refreshing your proxy list, because there are 100's of other scrapebox users using the same proxies at the same time.
I am am using a not so great list, and I just want to blast out say 10000 comments, I will use a public proxy and just post until I get 10k success.
If I am using my "premium" sources, posting autoapprove d0foll0w comments to high PR blogs, I use some "private" proxies. They are not exclusively private, but are shared with a limited number of users. It costs like $15 a month for access to 10.
When it comes down to it. "private" proxies ARE worth it, but they are NOT necessary.
For someone starting out, I would say a VPS is a better way to invest some money each month than private proxies, but if you want to make sure you get an excellent stick rate on high quality posts, use your privates.