Everyone including who started a wordpress site need to use a markup validated templet or theme. Among all the markup, HTML validation is most important to run a beautiful and seo friendly website.
To identify wheather your selected theme is validated or not visit at http://validator.w3.org/, put your web address and check. 
markup validation for wordpress
Now, carefully select your theme. if you already using a theme for long time, don't need to change it if there are a lot of error. Try to validate the errors if possible and as many as you can. Simply 5 to 15 error are not too much. Backup your data and database before starting validation works. Use a professional if possible.
Never hung only on beautiful looking wordpress theme. Consider seo optimization, HTML validation, site loading time factor, uses of flash, JavaScript, Ajax etc.