Your question has no answer. There is no "better", "best" or "ideal". It is completely dependent upon what specific equipment you are using, how your stereo is setup, your particular vehicle, etc. There is no one answer. Someone could be using the same identical speakers and subwoofers as you, but end up using different crossover points for their particular installation.
Set them to where ever sounds best in your vehicle, with your install, to your ears. That is the only answer.
General ranges; Subwoofers should *generally* be lowpassed in the 50hz to 85hz region, component sets *generally* highpassed in the 50hz to 100hz region. But there are, as always, good exceptions to those ranges. And your particular install may fall anywhere within those ranges...or even outside of them. But it should give you an area to start with.
