You have to decide when you think you will price it and ask the local elevator to set the basis vs. the respective futures month.  Meaning if you want to have until end of February to price it, then set the basis vs. March Futures, or if you want longer to price vs. May Futures will give you to the end of April, longer yet to price vs. July Futures will give you until end of June to price.

If you are truly concerned with the spread, you need to have an opinion on the spread.  Are you bearish the spread, spreads to widen, or are you bullish the spreads, spreads to narrow?  If you don't have an opinion of spread movement (the price difference between one futures month and another) or you don't have a clear plan of when you will or need to price, I would go out as far as possible to give myself the most time to price the basis ct without having to roll it.  So in the case of harvest delivery, the most time to price would be to set the basis vs. July futures.

When you set the basis you are drawing a line in the sand.  On the day you set the basis the elevator is going to look at where the spread is trading at.  So if you want to delivery at harvest and have until the end of June to price the elevator would look at the Nov July bean spread and add that to the basis.  For example, if we use the -15 basis at the start of this question and look at todays spreads on the CBOT, the Nov July bean spread closed at -16.5¢.  Meaning Nov futures are 16.5¢ lower or less than the July futures price.  In this example, the elevator would write the contract for harvest delivery at -31.5¢ under July futures and give you until somewhere between first notice day of July futures and the end of June to price the contract (mid June to end of June) depending on their policy.

In reality -15¢ the Nov basis  is equal to -31.5¢ the July basis if you do the math.  You are merely putting the spread into the basis to give yourself time to price the contract.  Deliver at harvest and have into June to price.  So is the 15¢ a cost to you, sure there could be lots of arguments, but at a starting point the answer is no because -15 Nov is equal to -31.5 July.

Starting today -15 SX5 basis:

is equal to -17.5 SF6

is equal to -20.5 SH6

is equal to -25.75 SK6

is equal to -31.5 SN6

Yes what the spread does after you draw the line in the sand can cost you or make you money.  If you go clear out to July and the spread narrows/tightens it would have been better to keep your basis vs. a front month and then roll it later.  On the other hand, if the spread would widen over time then going to the July upfront would be the better move. 

So by asking if rolling can take a bite out of your returns the answer is it depends on what the spread(s) do overtime and what futures month the contract basis is vs. 

I would look at what % carry the market was trading at and then try to decide how much spread risk (¢ move either direction, spread to narrow or widen) and make a decision on which futures month to set the basis against respective to when I plan to price the grain/contract.