I think it's always good to have a scratching post around. They aren't too expensive. I had an outdoor cat growing up and she occasionally scratched furniture...not as much as an indoor cat would, though.
My cat spends most of her time indoors but with occassionally be sent out by my SO
She has a scratch post as sometimes she claws at the carpet.
From stuff I've read on other websites, i would suggest having one if alot of time is spent on grass or muddy areas as concrete can help sharpen claws.
If the cat spends time inside the house, its always a good idea to have a scratching post - minimum of 4 feet high. Or you can bring in a log and he should use that.
When my semi-feral cat comes inside, the first thing he does is to walk up to the scratch post and give it a good scratch. It's not a bad idea to have one handy.