I have been selling my bitcoins earned from signature campaigns for more than a year now and never faced any problems with it. I sell them to local merchants in Localbitcoins.com and get the funds in my desired bank account and then i cash them out on ATM. There are no regulations regarding Bitcoin in my country and maybe that is the reason why i was never bothered by the bank or any authorities, but someone might have to answer a few questions if there are regulations implemented on Bitcoin in their country.
This may be ok if you are just dealing with small amount of sales, but let's say in 10 years 1 BTC is worth $1,000,000
How do you cash out $1,000,000 or any other amount near that? you can't do it through localbitcoins, you would need a million different merchants to cash out that much money.
Also there's no way to buy a house without going through the taxman. You may be ok cashing out a small amount to buy a new phone, but im talking about buying real state with bitcoin gains. In this case, you are going to need to explain the origin of your coins, and im not sure how I would explain I made some BTC posting in some internet forum, this may not be convincing to them even if it's the truth. They are too paranoid of anything bitcoin related.
This is also the area I'm looking at OP's worries - huge sum of fiat money transferred into an account, especially where it is above the stipulated amount for individual account.
Meanwhile on the other hand, I think to convince the authority about where the source of the money came from wouldn't be a difficult task, the authorities have a way of verifying facts because this forum is legitimate and some authorities are away of it themselves.
The point however is, if huge money transferred to an "unmerited" account as regarding specified amount for an individual account, it would then raise the curiosity of the government.