Sorry, but this is wrong. Sure, from time to time you've had a futures bet a double-figure odds and you'd like to guarantee yourself some cash, or thrown some money on a "for the hell of it" parlay which you didn't think would get close, and, again, you'd like to see a bit of that cash.
But saying "always hedge on the last leg of a parlay" is using flawed logic. You are effectively paying vig on both sides of the wager.
Using a basic example. So you've taken a four team parlay with an Australian corporate this weekend, taking four teams at the line, for $100.00.
So at 1.91 x 1.91 x 1.91 x1.91 you get odds of 13.31. $100.00 x 13.31 = $1331.00.
Your last leg of the parlay is St. Kilda -17.5 (wrong side IMO, but that isn't the point). So you decide to hedge on Collingwood +17.5. S you bet $697 on Collingwood +17.5, guaranteeing yourself a payout of $1331 for an outlay of $797.00, ie. a guaranteed profit of $534.
Now if you'd completely left out the MNF game and taken just the first three legs, you'd have gotten a payout of $697.00 from $100.00, a profit of $597.00.
As I said before, there is going to be the occasional situation when it is probably a good idea to guarantee yourself some dollars. Or, if the outcome of your parlay has had its probability altered (injuries, weather etc.) and the odds/line has dramatically changed. But saying to always lay off is wrong. What you have effectively done is reduced the odds of each leg from 1.91 to 1.85 (1.85 x 1.85 x 1.85 = 6.34), which, as you correctly pointed out in the paragraph earlier, is not good betting practice.
If you know you are not willing to take on the risk in the final leg of the parlay before you bet the parlay, then simple. Do not bet the final leg.
I thik the point is, when it comes down to the last leg - you don't have to lay off to guarantee the same payout with either result, but you should lay-off to cover your costs.
In the example you posit, (although I can tell you Saints were absolute morals last week to beat Pies by 30+ points), I would still have laid off with Collingwood for +17.5 with probably $200. It guarantees I end up ahead either way, but still the weight of money is attached to my original bet.
In my book, how much you lay off on a last leg comes back to your confidence in that leg. In that example, as I said, I would have sworn the Saints were morals to beat that line - so therefore I would only have laid off to make a negligble profit with Collingwood beating the line, but still with a sizeable profit for the Saints beating the line.
If for example that was this week, with Saints (-35.5) against Bombers, I would probably go closer to $350-400 on Essendon (+35.5). As though I'm certain we'll win, the line will be harder to beat this week against a confident Bombers. (I still think we'll beat that line though).
So, no hard and fast rules about how to lay-off, but you should at least lay-off in that example to guarantee green - why would you throw away a certain profit? Now, that is dumb punting - nothing wrong with having a spread on the guaranteed profit you're going to make.
