said or say

It's an example of what is sometimes called the timeless present, i.e. dentists didn't say it at one particular time, it is their general opinion so they may say that whenever the topic comes up, rather than just August 14, 2008, for example. It's like "People say that water freezes". Water will freeze anytime the temp gets below 32F, and people might say that anytime the topic of conversation turns to ice.

It's not a direct quote, so you don't need quotation marks.

The reason, Jinmin, is that the traditional "rules" that are generally described/have been described to ELLs are not accurate. They don't account for how reported speech is actually dealt with by native speakers.

It's isn't a "timeless present" as MJ suggested thought that certainly is one of the reasons that we use a present tense FORM reporting verb and that we don't "backshift" in the report.

[My other computer went all screwy]

It isn't a "timeless present" [Perceived general truth] as MJ suggested though that idea is certainly is one of the reasons that we use a present tense FORM reporting verb, or a past one, while retaining the same for used by the original speaker.

Jinmin's example is the State-event remains true[1]. This means that something that was recently said still is true at this time.

A: I going to go to China.

B: What did A say, C?

C: She says/said that she is going to go to Chin.

[1] The Grammar Book An ESL/EFL Teacher's Course

same idea, different term.

I don't think that they are, MJ, but ...