Reviews on wordpress twenty eleven theme ( free )

I am using Twenty Eleven theme on my blog. Is it good for SEO optimization.?

There is a update notification for this theme and it says

" Check the ones you want to update and then click “Update Themes”.

 Please Note: Any customizations you have made to theme files will be lost. Please consider using child themes for modifications" 

My query is : What if I don't use child themes.? Can I check and click the update themes button directly.? 

Note : Can you guys suggest me any good theme ( free ) for Phone related blog. ?

Thanks.