{{Quickfixn}} Missing tags, tag order; symbol tag 55 (mis?)listed as required?

[Manuel Lopez]

This is a minor point, but led to a few more general questions:
In customizing the fix4.4 definition file, I noticed that tag 55
(Symbol) shows as required for the "instrument" category, when it's
optional according to the fix 4.4 spec I have and our counterparty.
I'll add custom fields of course, but
1)  is it advised to check the xml definition closely for "required"
being correct and
2) for having all the fields in the same order as the counterparty
sends out (or expects)?  (I could for example worry only about those
issues that come up in testing, if I can assume that tag order is
rarely important, or only for repeating groups etc.)
3)  What is the best way to check for the existence of a tag value
before saving it to a database? for example, if I have (where msg is a
quickfixn ExecReport type):
TransactTime_fx60 = msg.TransactTime.getValue();
Can I just test for a null value?
(--I'm doing this in the OnMessage MessageCracker handler, assuming
that's the best place, rather than say overriding the store or log.)

Thanks (--and thanks for the great work in 1.5--SSL is just in time for me.).