Kragan,
I am not sure what CR will say but I would approach the UBQ quest with upmost prudence at your coin level. My current coin is at 3.2M and supply is at 4.4M. I read your post right before picking up my quest, and without doing any UBQ, I collected 228,332 supplies and 29,300 coins total. I have my SMB at lv4 and my CF at lv3. As you can see, it is easy to get carry away doing UBQ, I did at the beginning, but the amount of coins that I get as random rewards are actually very low, only 29,300 total. So, I would feel more confortable waiting until I have more than 8 millions coins/supply like CR suggested to start spamming the UBQ quest. Now, I only do it if in the previous random reward I get coins. I feel like at this rate, I can approach my target of 8 million quicker.