Yes, you can use PC boards for tie replacements. You can also use thin styrene like about .010 or .020. Either would be fine so use what you're most comfortable working with. As you say, once they are painted and the ballast is down, you wont really notice them no matter what material you use. When I've gotten really lazy, I just add extra ballast or weeds to the gap, depending on if it's a main line or siding and even that becomes barely noticable unless you're looking for it.
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