The Maccari springs and seals are very good quality. Those who go to the trouble of disassembling, de-burring the sharp edges, on the compression tube, cleaning, and relubricating -- in short, all the labor involved in a lube tune feel that for the cost of the spring and seal, it makes sense to go ahead and replace these while the air rifle is being worked on.
If the OEM spring isn't canted (bent) it is probably fine to just use the original spring in re-assembly. But the new seals aren't particularly expensive and many of the factory stock seals tend to be scored, especially if the were put in without great care and without de-burring the sharp edges on the compression tube first. So, it may make sense to replace the seal even if you don't replace the spring.
Finally, those who do more serious tuning than just the lube tune (as above) will actually custom fit a spring guide, add a "top hat", kind of like a very short guide at the top of the spring, and thrust washers (to eliminate the transfer of rotary torque and reduce spring twang. If you intend go to that much trouble, then it probably makes sense to use a Maccari replacement spring. They are made of very high grade spring steel and are machined and turned to more exact tolerances than the OEM springs in most cases. If you are going for the best possible tune, then you want to replace the spring, too. The Maccari springs generally have a longer life expectancy than the OEM springs.
