In most cases, yes.  OTOH just set it to 200 for most 'bright light'
areas and 400 or 800 for dim light and you'll cover most of what most
casual shooters need.  If it's really dim, then go to 1600.  How do you
know it's really dim?  When you get really long shutter speeds.

"A" means "Aperture priority" in which you choose the aperture (to
control depth of field) and the camera calculates the shutter speed
based on the meter reading (and meter mode).  Use the lens aperture dial
or the rear control dial to set the desired aperture with the "A"
selected.  In "P" mode the dial will reduce or increase the camera
selected aperture (and speed as a consequence) according to the meter
reading.

(On the D70s, the mode dial has an "Auto" as well as an "A" mode (MASP
block.  P11 of manual).  The "A" is aperture priority.)

This is the semi-automatic metering mode used by most photogs for most
cases.

Cheers,
Alan

    I guess that it depends on the type of photos you take.  If
you are planning carefully set up photos, with everything managed, then
you may want to manually control the ISO.  If you want to be able to do
quick shots before something photograpically good goes away, I would
keep it on automatic.  (I do that, and it does an excellent job of
selecting the minimum ISO needed to take the photo within the limits of
the set minimum shutter speed.

    Other differences:

1)    In [A]uto mode, the autofocus will select among the five focus zones
    whichever is the closest.  Sometimes this is what you may want,
    other times it is not.  In [P]rogram mode, The autofocus zone
    will stay wherever you selected it.  You can use the tilting
    poker chip to move the zone around on the display, unless you
    have the switch just below it set to 'L' (lock).  I tend to keep
    the autofocus zone in the center and the switch on 'L' most of
    the time.

2)    In [P] mode, you can use the two wheels (thumb and finger) to
    adjust either the shutter speed or the aperture, and the other
    will change to track it and maintain the correct exposure,
    allowing you trade off motion stopping shutter speeds against
    decreased depth of field, to isolate one plane in your photo, or
    to maximize the zone in acceptable focus.

    In [A]uto mode, you are in "Father Camera knows best" land, and
    you can't change any of those.

3)    As you noticed, in [P] mode, you get flash only if you personally
    opt to pop up the flash (or put an external flash on the hot
    shoe).  There are times when I will take a natural light
    photo, and then pop up the flash and follow it up with a flash
    shot of the same subject.  Sometimes one looks nicer, sometimes
    the other.  If you stick in [A]uto mode, you don't get that
    choice, because again "Father Camera knows best". :-)

    Note that I am careful to complete the spelling of "[A]uto", to
avoid confusing it with the other "[A]" setting "Aperture preferred".

    There are probably other differences between the two modes, but
these are the ones which I notice in normal operation.  I almost *never*
have the camera in [A]uto mode.  [P] does what I want, and allows me to
make fine choices most of the time.  I do also go to [M]anual mode from
time to time -- most often for the lenses which are too old to have a
CPU and to work with the TTL metering.

    If you are going to use [A]uto mode, the only thing that
distinguishes your camera from a Point-and-Shoot camera is that you have
the option to change lenses.  Everything else is taken out of your
hands, and put in the camera's hands.

    I would suggest that you play with [P] mode, and see what you
can get by playing with the values that the camera lets you adjust.
For example, the effect of depth of field as you change the aperture,
and the change in the ability to stop motion with varying shutter
speeds.  (And, there are times when you *want* some motion blur in parts
of your photos, to keep them from looking too static.

    Now to see what points others have made.

    Enjoy,
        DoN.