Concerning if item/supplier conversion is correct, Price column on PO does show correctly when using the base current_user.inc file, but maybe that is not an accurate comparison (because I don't fully understand the changes to that file).

I'm afraid I do not have any explanation.  The change to current_user.inc was the price_decimal_format () function.  The printed PO rounds using user_price_dec().  Perhaps you had to login/logout after changing Preferences->Prices/Amounts for it to take effect.   I don't know.  It would be silly, but you could just hard code the line in rep209 to use 2 decimals:

             if ($data['conversion_factor'] != 1)
                {
                    $myrow2['unit_price'] = round2($myrow2['unit_price'] * $data['conversion_factor'], 2);
                    $myrow2['quantity_ordered'] = round2($myrow2['quantity_ordered'] / $data['conversion_factor'], user_qty_dec());
                }

  I believe a manufactured item can still be used in a BOM list.

Looking at the PO print code that does the conversion back to supplier units,

                if ($data['conversion_factor'] != 1)
                {
                    $myrow2['unit_price'] = round2($myrow2['unit_price'] * $data['conversion_factor'], user_price_dec());
                    $myrow2['quantity_ordered'] = round2($myrow2['quantity_ordered'] / $data['conversion_factor'], user_qty_dec());
                }

it seems to me that if user_price_dec() is 2, it must round to 2 decimal places, so that is why I am guessing that you might have changed Preferences->Decimal Places?  Or maybe you don't have the supplier pricing/conversion set in the Item?

I would still prefer to enter a PO in supplier units qty units and cost).

  Thanks again Braath!

Change the includes/current_user.inc function to:

function price_decimal_format($number, &$dec)
{
    $dec = user_price_dec();
    $str = strval($number);
    $pos = strpos($str, '.');
    if ($pos !== false)
    {
        $len = strlen(substr($str, $pos + 1));
        if ($len > ini_get('precision')-3)
                    $len = ini_get('precision')-3;
        if ($len > $dec)
            $dec = $len;
    }
    return number_format2($number, $dec);
}

from the base

function price_decimal_format($number, &$dec)
{
    $dec = user_price_dec();
    $str = strval($number);
    $pos = strpos($str, '.');
    if ($pos !== false)
    {
        $len = strlen(substr($str, $pos + 1));
        if ($len > $dec && $len < ini_get('precision')-3)
            $dec = $len;
    }
    return number_format2($number, $dec);

I think the base code sees the desired precision ($len > $dec) but only sets $dec to $len if $len happens to be less than the php.ini precision.   It usually isn't so it uses the user decimals (which is probably 2, which causes the rounding problem you describe).

In my code, the desired precision is tested against the php.ini precision and truncates if necessary.  Then if the result is greater than user decimals, it uses that.  I think this is what the base coder wanted but just didn't code it right.

If that doesn't fix your problem, I will have to look deeper into how I fixed it.