Various ATC questions

A few questions on ATC:

1. If I am told to "depart XYZ heading 150" while IFR, and the last
heading given to me by ATC does not quite take me to an intercept of
the XYZ VOR, should I turn to intercept the VOR? Should I wait for
ATC to tell me to turn directly towards the VOR? Should I turn and
apprise ATC of my turn? Or should I just intercept the 150 radial
from the VOR, and then turn to follow that? The last time this
happened, I just silently turned to intercept the VOR (I wasn't far
off, and I assumed that ATC wanted me vectored towards the VOR to
begin with, since it was in my flight plan and I had previously been
vectored in that general direction), but I don't know if this was
right.

2. If I am VFR in Class B, on a transition route, are altitude and
heading at my discretion (within the limits of the transition route)
UNLESS ATC directs me to a specific altitude and/or heading, or do I
always wait for ATC to provide exact instructions?

3. If ATC says "proceed direct to XYZ," and XYZ is a waypoint in my
IFR flight plan, do I assume that I can resume my own navigation after
reaching XYZ, or do I maintain the same course and altitude until
otherwise instructed, even after reaching XYZ?

4. If an IFR departure plate says "fly runway heading, then vectors to
fix (or) XYZ," I take that to mean that I continue on the runway
heading after take-off, until ATC tells me to turn towards some other
fix. Is this correct? If the fix is on my flight plan, do I
automatically resume my own navigation after reaching it, or do I
maintain my heading until ATC says otherwise? If the fix is on the
plate, and the plate says "then own navigation," do I resume my own
navigation after reaching the fix, or do I wait for ATC instructions?

As you can see, I'm still confused as to when I must wait for ATC
instructions to change altitude or heading, and when I'm at my
discretion to do so (either without restriction, as in VFR, or
according to my flight plan in IFR). Recently I went too far in a
Class B airspace in my sim because I had been told which runway I was
being given, but I had not been told to turn, and I thought I had to
continue in the same direction until ATC explicitly gave me a new
heading. So I'm still confused.

Cheers,

Craig