Good question. I have a Sora equiped 8-speed that I was planning on upgrading when the parts wore out. It is my daily ride and I've had it for 5 years. I got it as a "rain" bike, and it ended up being my favorite bike. I've changed the RD to a Mtn bike RD because I'm running a 12-34 cassette, so I don't know how good they are. Nothing else has worn out yet. It shifts better than my touring bike with Tiagra, and my "good" bike with 105's! It seems like a lot of the touring bikes in almost all price ranges come with Tiagra FD's. I suspect there is a reason for that.
If your bike is shifting well, I'd just wait for the parts to wear out and then repalce them. However, if you have the smame experience that I have that could be awhile.