Im with Andy, I always make the isolation as BIG as practical. If you have to pack it in, you have to pack it in but only do so where absolutely needed. You can pour different polygons with different isolation on the same net too if you need it tight one place and not another. Every time i've gotten bad boards from OSHpark (3 times this has happened) it was because there was a super tight/dense space and the etching didnt get all the copper. They are probably way more sensitive to this than other fabs since they do big ass boards with many designs and i bet its easy to have issues etching something that size.
I think it boils down to getting a handle of what the fab says they can do and what they can actually do reliably. I had problems with Seeed with my first order there and never went back, the etching was rough, the soldermask was poorly registered, just blah everywhere. JLC i'll use for big stuff since their pricing is good but small stuff they have issues with clean screens even at their minimum text size spec. PCBway has nice machines but they take a maximum X size of 110ish mm so big stuff gets pricey there too, They cant do small text like OSHpark can on that machine. So it's a mixed bag always where ever you go. I go through a thousand boards a year and have yet to see a etching fault on any of the Chinese fabs which is why I dont use OSHpark anymore, it was a convenient service 5 years ago but the prototype biz in china has exploded and you can do much better for much less time and money. Standard turn around for protos is 2 days at any place out there