Okay the placement of your widgets and ads on your blog/website are very important for both you and your visitors.
You need to consider ease of finding the ads but balancing not making it to easy that they can ignore your content.
I get alot of people people who put their ads at the very bottom of the page, which I find quite annoying if I dont enjoy their content and I am visiting while dropping.
I personally think that you should put your ads just bellow the initial scroll area. This means that your ad is not the first thing they see, they cannot get too tempted to click and leave. But it is also close enough that if they do want to leave they can easily get to it after a quick search.
This is also something which is good for regular visitors who read posts on each visit. They will be able to quickly read the first post (probably their only unread post) and then quickly access their way out.
You may think making it easy to get away is going to make your site bad or get less visitors but if the site is easy to view then your users will have a much better chance of revisiting and enjoying their stay.
The best thing to do is to imagine you are the visitor and see what you think needs to be changed. Then change it, I do it when I design websites. A client will say i want : A C A D B but i will know from experience that B D A C A will work 10 times better. I will normally try to make a quick version of each to show the client the ideas then work from their.
Customer (visitor) knows best but it is always good to do what you think is right also.