Related to the other thread about getting old caked on grease off of bearings, I'm now wondering what is safe/best to use to clean the same caked on grease off the rubber seals in the headset (or any other rubber pieces on the bike)? I've been reading that the mineral spirits I used on the bearings may not be ideal for the rubber and may cause issues. The citrus degreaser I'm using doesn't specifically list 'rubber' in its list of things not to use it on, but it's also not included in the list of things it is 'good' for.

Any suggestions?

Various types of plastic and rubber can be damaged by various solvents. There are some degreasers out there that claim to be safe for use on most types of rubber and plastic, but I usually use liquid dish soap like Dawn or Palmolive to get grease off of things that could be damaged by the strong stuff.

It depends on the type of rubber. Since bearing seals are usually made of synthetic rubber and live in contact with petroleum products, standard degreasers should be safe. This would include mineral spirits, brake cleaner, and carb cleaner.
Hydraulic brakes may be adifferent story since some materials used with DOT fluids react with petroleum products and solvents, however automotive brake cleaner is usually safe here.

I usually just use a dry rag, or Qtips. Roll it off, rather than wipe it off. It usually comes off pretty easily, and is sparkling clean. I never use any kind of cleaner - it just comes off!

Products | SuperClean

This has been the only thing that will take my car tires, strip off all old armor-all, and leave a brand new rubber look.
Some hot water and a brush and this.

Find it at most stores.

Brake fluid or ATF.....

WD-40 is an excellent cleaner for grease and softens dried hardened rubber. That and the smell is fantastic.

warm soapy water to clean. rinse with clear water. clean/cheap/readily available

I have seen this product take paint and chrome plating off...................... be warned.