Mainly its for the Services no matter what but it is required in other sections if your IP is soft blocked as mentioned above. If you allow your IP's to get noticeable by posting to many ads, then a soft block will occur on your IP or IP Ranges. You can try changing your IP yourself if your using DSL or Dial-Up and you may be able to dial into a descent range to post in. Most exclusive proxies that I"ve tried and currently using will bring up the PVA block right away depending on if other people are using either shared proxies or mass posting on CL with a close IP range to yours.
The block lasts about 2-3 days from my experience.
For the question regarding the 10 accounts, the most that would happen is that they would be destroyed and you wouldn't be able to log into them. If that happens, which it will eventually, then just buy more or make a few more. If you blasted thriy posts to thirty cities in 30 minutes is not that bad either. I personally do about 2 ads per minute without problems. So yes ghosting will still happen every once in awhile regardless, so just stay on top of your ads and the more variations you have the longer you'll post without getting ghosted. Eventually things do run out such as titles, image hosts, and will start causing ghosting; just input new url's or content as soon as it starts happening.
For the proxies; I believe your thinking about the accounts. You can post 1 PVA 1 ad per 48 hours. This rule only from my own studies applies to the Services section. I can go into the other sections that are just IP Softblocked and use 2 PVA's to post 100 ads. I usually do 2 PVA's for every 100 and alternate every other day. But in the Services, you must stick to the 1 PVA 1 ad per 48 hour rule. I do this along with proxies and I can get away with about 8-10 posts per proxy IP per day. Of course the best strategy is to request multiple different classes and especially class C's. If you can also find a provider who will give you fresh IP ranges monthly, then thats even better. Some will do it, just have to ask them.
I hope some this info helps you.