I would definatly say near enough is good enough.
If wastegate selection takes 10 days to calculate your engine will never be finished LOL!

As a rule of thumb, to be on the safe side - meaning no boost creep and good control, my personal recommendations would be:
2.5 - 2.8 l = 38 mm - 48 mm
>2.8l = 44 mm - 60 mm

 

As they say - the devil is in the details.

I usually spec the bacon first....

From a 'real world' perspective - I have not seen boost creep in a properly set up tial 35/38mm gate. Keeping mind that you would only see this at very low boost settings. I would say that if you want to run a 3.0 at less than 1 bar you may start to get in to an area that 'needs' the larger gate. Important to note that I prefer to run slightly smaller hot sides than most to keep up a quick spool and big mid range torque, this limits the max top end boost. If you are building a dyno queen/mondo turbo engine you can safely ignore me!!!