Yes, when you already have discretionary income, it would be better to start investing in bitcoin as soon as possible, so don't wait too long. But in my opinion, waiting one day is not a problem, although it is better to do it as soon as possible. Because basically why should we wait if we already have discretionary income. Maybe if we don't have discretionary funds, then we should wait until we have discretionary income. But if we already have it, I also feel that buying bitcoin directly is the right step. But everyone is different and has different self-confidence. So for people who are not too confident about investing in bitcoin, they will definitely feel a little hesitant to start investing.
You can start your investment in Bitcoin even you don't have any discretionary fund at the moment because everyone have different beginnings. Even they are not good in finance, if they have income that can be used for investment, they can start their investment adventure. However, they must manage good balance between their spending and their investment capital as well as never forget one part for emergency fund.
Through investment in Bitcoin, their finance will be improved considerably with time, and they will gradually build up both their wealth and discretionary fund that can be used for investment more solidly. Not everyone can start with descretionary fund for investment and even this strategy is good for financial management, you can start with not too good finance, and improve it with time. The vital point is always have emergency fund that plays its role of making sure that you will not sell your bitcoin when you don't want to do that.