I tend to view it like, if I'm masturbating and thinking about going for a spur of the moment punt, but end up wanking to completion instead and then not having that punt, then I didn't really want that punt in the first place.
It can be a bit like a test I suppose. How horny am I really? Will a wank be enough to satisfy me? The answer, for me, is very often, yes.
If I am absolutely dead set on having a punt though, then there's no way I'm having a wank any closer than about 10-12 hours before. I'm saving that spunk for usage during the punt instead, especially if I'm going for an hour punt with a decent indie girl, want to try my best to get two pops in and a good energetic fuck, that's something I need as much sexual energy built up as possible to do.
That's not to say though, that I've never had a wank and then gone and had a punt not long afterwards, and it doesn't necessarily completely ruin my enjoyment. Some prossies are very good at dealing with punters in that kind of bit tired, just had a wank, kind of state. A lot of parlour punts come to mind here actually. I'm not necessarily expecting the best punt ever out of them anyway, so I have often just casually strolled along to such a punt in that kind of post wank but still a bit horny state and just sort of taken the resulting punt for what it is.