The results of Are you seeing ads on Quora? by Nick Nicholas on Assorted Polls as of this writing:
79 answers received. 13 have never seen an ad, 66 have.
Of the 13 who have never seen an ad, 6 are current TW, and 7 are not. Of those who are not, 3 are in North America, 2 in Europe, 2 in Asia.
Of the 66 who have seen ads, 1 is a former TW, and the remaining 65 are not TW. Of the 66, 10 are in Asia, 6 in Europe, 41 in North America, 5 in Oceania, 2 in South America, and 2, somehow, are Other.
This confirms that Top Writers are not seeing ads; the immunity of Former Top Writers, which has also been reported, is not necessarily universal. No clear correlation with region in visibility of ads.