I Heard the same, use it or loose in. There is nothing wrong in masturbating and supposedly you make a fresh batch of new cum which is healthier...
I think that probably depends on how long it has been. Immediately after an ejaculation, to start with the number of viable sperm will start to increase. If you don't ejaculate for a while, eventually there where come an equilibrium where the number of sperm finishing maturing and becoming ready to go it balanced by the older ones dying off. I don't know for sure how long it takes to get to that point but given that labs that do semen analysis ask for three days abstinence it may be as short as that.
I am not sure if that means there is an ideal frequency for pregnancy, though. If you're trying for a baby with your wife then basically the few days up to and including ovulation are the optimum time but whether there is any difference in "drip feeding", i.e. having sex every day or every other day so she gets whatever you made so far, or saving up and putting them all in once I don't know. One of our fertility experts, Lord Winston, just advised "Have lots of sex!"
Then, of course for most of our lives we're not actively trying to get someone pregnant, whether it's pre-family, family complete or don't want a family and during that time there is no need to worry about any of this. If you're horny, would enjoy cumming and feel better afterwards then go for it with whatever means are available to you, i.e. whether that's sex or a wank.