This question is dificult to answer. But my short answer is no.

Do not trust all plastics to be white at all.

Do not trust glossy plastics.

Do not trust transluscent plastics.

It is dificult because it is a case by case answer. It can not be answered inclusive by generic answers, for example pvc pipes of diferent brands have noticable diferences.

Some styrene I have shooted has a very noticable magenta tint, some formaica has a blue tint.

Plastic can be afected by age, solar exposure, oxidation which can give a warm tint.

Inclusive the nylon used in softboxes can turn yellowish in time.

The main concern for a plastic manufacturer im sure is cost-benefit-properties (Temperature and solvent resistance, degradation, translucency, etc). Some of them sure, are meant to look white enough, but I supose NO one takes into an account that some photographer will see the perfect white balance of a plastic to be used as a background or white card.

So the answer is no.

If you are looking for a cheap white bananced card I supose they have a minimum quality to offer somehow neutral white. Probably they have a white paint coating. Paint is more likely controlled to be white. One main purpose of paint is to be somehow acurate in color.

If you want a cheap option you probably could go for a high quality inkjet bright paper. I would trust more on this paper than almost any plastic.

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P.S. There is no white 255,255,255. That is a blowned white. Almost all colors on the planet will give you that color if you leave your shutter open long enough.