When does sql optimization become redundant?

Question

I am updating tables with millions of records and I need to be as efficient as possible. Does it make sense to add more criteria to the where clause rather than help?

For example, if you know, I want to set the column to 3, I could use this query:

update mytable set col = 3

Or I could update the record only if it is different

update mytable set col = 3 where col <> 3

I could also filter it so that it only updates records added since the last start of this process

update mytable set col = 3 where col <> 3 and createDate > @lastRunDate

And maybe I could look for extra things in extra columns.

I think my question is, is there a point where the cost of finding additional columns outweighs the cost of the update itself and if there is a principle that you can use to determine where to draw a line.

Update

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