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The 10% guideline is a good one. Derating to operating at just 75% of rated voltage is much more conservative than what the original designers did.
I find many sets exceed the voltage ratings of the capacitors during warm up, and they seem to survive just fine. Usually electrolytic caps have a surge rating which is higher than the working voltage. Although it is stated above that a 5U4 or a 5Y3 has a controlled warm up speed, I find they still warm up and produce voltage 2 or 3 times faster than the rest of the tubes (with indirectly heated cathodes) warm up, so even sets with tube rectifiers have a big voltage surge during warm up.
Although electrolytics warm up very slightly when in normal use, I would say most people almost wouldn't notice it. If it feels obviously warm within 10 minutes or so, something isn't right. However, make sure it isn't simply being heated up by nearby tubes. I agree with the statement above that electrolytics used in voltage doublers have much higher peak currents and will warm up a little more.
If yours are getting quite warm, I'd look for something other than just a surge voltage that merely approaches the working voltage temporarily during warm up.
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