Is the Cantonese spoken in Guangzhou standardized? Is the Cantonese spoken in Guangzhou standardized, and if it is, is it different from Hong Kong and Macau's standard Cantonese? Do some characters have different standard readings in each region, much like how some characters have different standard Mandarin readings in Mainland China and Taiwan? The closest example I could find for Cantonese is the character 「米」 when meaning metre. Below is the explanation given on the Yue Chinese Wikipedia:


  米... 廣東地區讀mai1；香港讀mai5


If Cantonese in Guangzhou is indeed standardized, is there an authoritative source that decides on the standard reading of characters? I have found dictionaries published in Mainland China like 廣州話正音字典 and 廣州話方言詞典, but I don't have much insight on how close those dictionaries are to standardized Cantonese in Guangzhou, assuming that exists.
It doesn’t seem like there is an organisation for standardization of Cantonese in Mainland China. After all, this is in line with their policy of Mandarin-only education; people throughout the nation are not fluent in their native dialect, and can only speak Putonghua. 
In contrast, there are governmental agencies in both Hong Kong and Macau dedicated to the standardization of Cantonese in those territories. 

It is worth noting that the HK-Macau standards originate  from the pronunciations of downtown Guangzhou speakers. If a Hongkonger were to visit downtown Guangzhou, he would probably face no problems communicating with a local there.

Branching out of the provincial capital of Guangdong, though, there are other accents which are vastly different than that of mainstream media (i.e. HK-Macau-Guangzhou accent). For example, in Shunde, which is just beside Guangzhou, they pronounce their words such that learners of Cantonese might think they were purposely trying to mispronounce the words. 

Taishan, which is also within the Pearl River Delta region, is also famous for the Siyi dialects which are related to Cantonese, but are mutually intelligible. 
In HK-Guangzhou, you would say “ngo” for 我, yet in Taishanese it’s something like “ngoi”.

Of course, you also have pronunciations influenced by Teochew and Hakka, the other two indigenous dialects in the province, as you approach the edges of Guangdong.