Depending on what side of the spread you are on it can help or hinder. For instance Mark is very good at making a book and would rely on getting the prices on the right side of the spread (and before or after movements).

I am not good at that and so I rely on the final figures and usually (not always) am on the wrong side of the spread but I can make it work and it suits me. To me the difference between offering $200 vs $400 is so small that its not going to erode my profit margin too much. When you think about it for a $100 liability the difference is only 25c. If your relying on that 25c to be your entire profit margin then your in trouble. The difference on a 10,000 outlay is only $25 and I would be expecting a much larger profit then that after risking 10K.

My problem with backing and laying is the time it takes to place all the bets. By the time you have placed a bet on horses 1-5 the bet on horse 6 is probably at least a few seconds later than the price you last checked so your behind the latest information already. This might not seem like much and probably won't be an issue in most cases (especially if your using SP on unmatched bets) but what if the race goes in play before you get your bet on and its not taken ? It might only happen 1 race in every 100 but that might be a large percentage of your profit margin.

So I would prefer to go the method with the least transactions whether that is laying or betting (usually it will be laying).