When an advertisement that is related to your content, or is related to the niche that you maintain on your blog, whether it is in the sidebar, whether in the header or anywhere else, then you give your readers Providing a place where they are getting to know about a product or service that they might be interested in buying.
For example, here on ShoutMeHindi, we mostly write about Blogging, Web Hosting, online earning money, etc. We also share our personal views and experience. Many readers here are just for this because they love to know about my experience and they want to know which products I use and maybe they want to use products similar to that. Huh. They are reading my posts because they have some interest in reading this niche, which I write. So if that is the reason why you see banners of web hosting etc. on our blog, yes, in sidebars or in headers.
If you have an interest in your blog, then there is no right or wrong time to start monetizing it. If you are in a dilemma about the things written by your own blog, then perhaps this is why you will be apprehensive about monetizing it. Keep in mind that to place AdSense ads in your blog, only two or three places on a page are good. And it also happens that low ads = good CTR = assurance of earning good money in the long term.
So if you have an AdSense account and a blog, try to start placing some ads on your blog. Start with two or three ad units, and take care of the quality of your blog. If you provide good information to your readers, and overall keep everything good, then it is your benefit. What do you know that your blog should become the biggest way to live your life!