I think the only reasons to cash out is if you no longer like the chances of the horse, whether it be a change in ground conditions which wont suit or maybe if the pace horses are non runners and your selection needs a good pace to run at or something along those lines, even then it's still tempting to let in run.
The feeling of missing out on that big win because you cashed out is far worse than letting it run and the remaining horses losing, you're still gonna pick up some money.
I always think I had my reasons to pick them in the first place so there's no reason to change it, you've still got to trust your methods as you would if betting singles.
I'm talking lucky 15 31 etc, I don't really do accas but I guess the cash out would be tempting if it was down to the last selection and its either zero or a decent amount in your eyes, although you could probably get better deal buy using the exchanges.