I believe the big tag has been depreciated for quite some time. Tags like this are generally ignored by search engines. A better way is to use h tags or if you are using html5 you can use hgroups and then applymthe style through CSS.
I do not prefer to use tags. If I am doing SEO, I use this strategy: 1. I put the most important keywords in my title. 2. I put keywords in my headings. 3. I evenly disperse keywords throughout my text. I do not keyword stuff because that will also have a negative impact. Keyword stuffing is when you try to cram the keywords in your text as many times as you can. You must include the keywords in such a way that it is natural and grammatically correct. This way, your content not only will still appeal to your audience, it will not be rejected by the search engines. I hope this helps out.
This is foolishly untrue. Search engines depend upon tags to let them know important details about your content. If your content does not have title, description and heading tags, your SERP will be very poor.