Though I'm a fan of the first "age", before the movie(s) and cancellation. I heard bad things about the later seasons, and don't want to taint my memories of the show by watching them.
I'd suggest looking at episode ratings or something and cherry-picking. It definitely was not some sort of total tanking or anything.
In fact, I'd call it the only case I've ever seen of trying to "bring the band back together" that largely succeeded. Not 100%, no, but they did much better than average.
That said, when the show ended for the last time, I was satisfied.
IMDB ratings seem to agree with your sentiment (7ish out of 10 opposed to the 8ish of earlier seasons). But watching through the series start to finish I didn’t notice any drop in how much enjoyment I derived from the show. I definitely don’t think it will taint your memory.
And as another sibling post said, the send-off episode always tugs at my heart.
I’d recommend finishing the series if you enjoyed the earlier seasons.
The later seasons have few worthwhile episodes, notably:
* S6 - EP7 - The Late Philip J. Fry * S7 - EP26 - Meanwhile (Series Finale)
That said, I agree with you, most of my favorite episodes are from the first four seasons.
There was also an amazing song: https://www.youtube.com/watch?v=LE1drY3A418
People who did not know Futurama found it puzzling, but it was just so funny I could not resist.
Also, I think the "second age" of Futurama was as good as the first one. I loved the series finale.