make sure all your seo names are as you want them first, usually duplicates of product title. You can check them by going
admin>export>products, then just choose code and seo field., Check them all in a spreadesheet if you need to, and import any you have changed.
Turn it on as soon as possible as whrn you do it will change product_id12345.html to yellow_widget.html and if google has already crawled 12345 then you will get 404 for that product. though these can all be 301 redirected if needs be and cs cart rocks canonical addon can help here
go to webmaster tools and download your 404 table, then you will have the ones google has already crawled, a few 404s arent that bad, remember to re submit your sitemap in WT after generating a new one