One of my blog was recently de-indexed may be because i had used long titles.
That's probably not the reason you were de-indexed. Long titles by themselves are not something to remove pages from the index for. I am sure there were other, heavier weighted, issues that caused your problems. Low quality content is the first thing that comes to mind.
Can some one tell me if its okay to use such keywords in the beginning of post ? Is it allowed by Google search engine guidelines? is there any chance that the blog will get de indexd or penalized ?
Tags are fine to use when used properly. Generally speaking, an article or blog post will have a handful of tags that apply to it. Those tags will link to a main tag page that lists other articles that also use that tag.
If you have a dozen or more tags for each article then you might be using tags incorrectly or need to break your content down into smaller pieces. If your tags do not link to a main tag page, or do not link to anywhere at all, then they are being used incorrectly and may be problematic. Especially if they aren't links at all. That just becomes keyword stuffing which is a no-no.
Common practice says that tags belong at the end of an article or blog post. Unless you have a really good reason to put them somewhere else I would stick to that convention.