I have been using the same rig, already set up on the same length of mainline for about three weeks now. I have fished about eight sessions.
I have caught a number of Carp to 20lb, and a good few Tench.

Although I check the line carefully for any abrasions, should I still strip the rig down and cut the mainline back to start afresh after each session?

What other reasons could there be for rig or line failure which are invisible to the naked eye?

a lot depends on the rig, the set up and the venue.

Nylon or flouro with a knotless knot will flatten and weaken where it exits the eye after some stretching, this severly diminshes the knot strength.

Chaffing around beads, knots, run rings etc should be checked and replaced.

Hooks, especially over rough bottoms should be sharpened or replaced after every cast.

I've started doing this after losing a nice fish after having landed a few on the same hooklink. Whos to say it wouldn't have broken anyway but I won't take the risk from now on.

Fishing the Derwent on Sunday I had to replace the hook length almost every cast due to abrasions on the flourocarbon hooklength. Other times I have checked and used the same one for a whole session landing several and every fish.

Thanks,
there are some good points there.
The safety aspect goes further than the hooklength though.
How many people bother to check their mainline properly?

This means checking it further than the length of the rod. My usual casting distance is around 30-40 yds. How do I know if somewhere in that measurement on the mainline that it hasn't rubbed against a snag somewhere?

I suppose the only way is to ask somebody to reel in for you while you check the 30 yds or so of mainline. I don't think I have ever seen anybody doing this.

Most anglers say they change the mainline once or twice a year. But is that really good enough, without giving it the once over now and again?

If you buy a bulk mainline you could change it when ever you feel it needs it. Or if your not sure about anything. On one reel I've been using daiwi sensor and I've respooled it alredy this season as I ended up in a few snags.

Ahh but they were obvious snags maybe Christian. How about the little nicks we may not notice when a good fish is pulling your line?

If occasionally when you reel in, hold the line between your thumb and forefinger, particularly at the end of the day. Have one last long cast just to prove it.

Your finger can detect a blemish (nicks) in a surface of 1/1000th of an inch or less than the half the width of a fine human hair. Any abrasions or kinks show up like your tripping over a log.

I retie the knot to my Gemini clip (what the hooklink swivel is clipped to) every couple of trips or if I haven't used the rod for a month. It costs maybe 4 inches of line, or 1 yard every 9 trips.

I see what you mean by your thumb and finger detecting any blemishes. But at times, you would need to reel in pretty fast to stop getting snagged on the lake or river bed.

There are obvious and reasonable checks you can do as described above but sometimes there will be a nick or stretch or kink or something that gets missed. All anyone can do is take reasonable care, I think it will or should be obvious if more care is needed by an individual through the number of fish they lose.

Touch wood, I haven't lost any yet. And that's how I want to keep it.

This seems like a Baz wind up to me , the same rig on the same mainline for THREE WEEKS ,your telling pokies Baz !