Only if they have transparency, and even then not necessarily, though it certainly will tell you what is going to interact. I wouldn't hesitate to check the overprint preview, though.
By 'sending to print', do you mean sending files off to a printing company? If so, it depends on their file requirements. For instance, if it's PDF/X-1a, you'll be flattening the file at your end, so you'll be able to see how it flattened in Acrobat. If it's PDF/X-4, flattening is their responsibility.
I can't remember the last time I used Flattener Preview, but I use Overprint Preview all the time.