Replacing tubes on used equipment bought on A'gon

Over the last 18 months, I have bought through Audiogon a CD player, preamp, and amp...all tubed and used. They are not giving me any trouble that I know of. As I am a newbie to tubes, is there any merit or need in replacing the tubes on used equipment that is purchased since I don't know how they sounded as new? Thanks.
Depends on type of tube. Tubes degrade gradually with use, but will "work" considerably longer than their ideal useage time. So you could have a tube working 5-10 years that musically degraded after 2-3 years. The only way to know is to try new tubes and see if you hear a difference. If not, you have a back up set of tubes.
Investing in a tube tester may be a good idea if you have a lot of tube gear. If you have a local club they too may have a communal tester that you can use.
I would find out what the manufacturer requires for stock tubes and purchase a set for each. This way you'll have a good idea of the voiceing intended. From there you can descide for yourself whether its worth the tube rolling or not...Have fun !