Is the Cantonese spoken in Guangzhou standardized?
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.