[CSS][SEO] headings

I noticed that headings (H1, H2, …) are different sizes on Firefox and Internet Explorer. Is it worth the time to create an extra stylesheet to fix the size discrepancy in IE or should one just not use headings? As far as I know, most other tags come out as the same size in both browsers. If I were to not use headings to save time, would it hurt the search engine optimization?