I think it's just fair to pay for the extra task of writing for meta tags, SEO title, and description -- since it's extra work for the writer. I for instance, am paid for the article title and body, but extra for these tags. Unless you're paying the writer very high already, that could be part of the package.
Writing is writing- pay the writers for their time (I may have ulterior motives for propagating this attitude....)
I always give something before he is starting just for motivation, and the rest after the job is done and all works properly.
Every freelance copywriter will have different ways of doing business. If you like their work, or need their expertise, you should pay them how they tell you to! Of course, that's a copywriter talking. But I know that, in my case, I'm acutely aware of what quality copywriting is worth to a business. I feel like I should be paid in line with that. As for the 'per hour' thing, I think it can lead you down a difficult path. Should I invoice people for the hundreds of hours it has taken me to build up my skills? The hours I spend doing my books or contacting new prospects? Fixed fee for me. I guess the simple answer is 'If your copywriter charges you extra for meta tags and he/she is valuable, pay up!'
I would say that if you do an hourly rate, you would bill them per the hour you are working on their project, depending on your skills gained from the hundreds of hours to build up your skills. You need to raise your hourly rate to a price that compensates you not only for the work you are doing, but also for your overhead- time spent getting contacts, money spent on rent/internet bill. If you are freelancing full time, this is your livelihood, and your hourly rate needs to reflect that. However, I too prefer to work on a fixed free basis as I feel it is more suited to the work.
Paying something upfront is good not only because you DON'T know the writer that well, but also as a reservation for the writer to set aside time to work for you...because more than likely s/he has other clients.