At one point or the other, we all need to buy bitcoin
No you don't "need to" buy bitcoin!
but when do you actually buy it? or when is the best moment to buy bitcoin?
Short answer: whenever you have some extra money laying around.
For example, should I save some cash to but bitcoin when there's a 0 -15% dip? or should I wait till there's a major drop of >30%?
It depend on whether you are buying bitcoin with a long term view or you are just day trading for short term profits.
If it is the former then you shouldn't concern yourself with "dips" that much even though it is preferable to do. The problem is that you may keep waiting for a >30% dip while price keeps going up so you end up not buying at for example $3200 while you watch the price reach $7000 then decide to buy!
If it is the later then you want to always wait for the best entry point because you would also exit soon after (to get the profit out as a day trader). But that requires a lot of market analysis and the size of the dip depends on market situation, it is not going to be a fixed number.
What if I invest in an altcoin that booms and then take profit from it to buy more bitcoin?
Normally you don't buy altcoins with money, you buy them with bitcoin and also normally when you want to dump them and get out you convert them back to bitcoin directly.
This way you avoid additional exchange or even withdrawal fees.