In my own opinion, it should be done at the earliest opportunity, but the major problem here is the amount involved, because $10 is too small to be doing that weekly because the fees alone will be reducing the fraction of the Satoshi bought.
So on that note, due to the amount involved, I think that the logical thing to do is to do it in his second acquisition, which is $20, wish might cost him only $0.50 in fees.
Since those acquisition will come from exchange it will be very high in network fees because exchange cannot give you $0.5 to withdraw $20 and this is also applicable to all the units you might want to withdraw from Bitcoin to your personal holding wallet. However this two times acquisition before withdrawals could have been better for someone who is investing high because when is deducted you will still have your main capital almost 100 percent complete because if for instance they have a network fee of $8 and an investor invested $2k, after the fee they will still have $1,992 but the person with $20 will be left with $12, so this is something that should be calculated and no how many times it should be withdrawn.