Order by desc as default option for SQL Server Management Studio?

There is no default sort order in SQL

If you are seeing "oldest" on top then that may be the fastest way for the engine to retrieve it because that is how it is stored on disk.

You are not guaranteed to get it in this order, consider it "unordered" unless you specify an order!

ORDER BY is the only way to have results in a specific order.

Ordering can be an expensive operation depending on the table and order specified, so unordered is the norm.

What JNK says is 100% correct.

But if you just want it to normally work, and only when you open a table rather than when you query a table...

Try adding a clustered index, with the first indexed field being indexed in descending order. This will likely actually cause what you need.

(If you already have a clustered index on that field, edit its properties and change its ordering.)

This is only a sensible idea if such an index is friendly to the actual use of the table. It would be self defeating to have an index that's useless programatically, just for your convenience ;)

You cannot change existing template for generating SELECTs from context menu in SSMS.

Thankfully, SSMS is extensible. This means that you can write your own extensions which can do exactly what you want, or you can use some existing solution. I would recommend Mladen's SSMS Tools Pack:

http://www.ssmstoolspack.com/

It was free until recently, and still is for versions 2008r2 and earlier.

Although officially there is no default sort order for the simple linear input I'm getting satisfactory DESC default sort order with PK or IX sort order. Let's say for log tables where I'm interested the most for last entries.

CREATE TABLE [dbo].[tableA]([DateTime] [datetime] NOT NULL,
CONSTRAINT [PK_tableA] 
PRIMARY KEY CLUSTERED ([DateTime] DESC)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]

Or in SSMS ...