There is also an advance technique called "STOPPLING" to do hot tapping and even rerouting gas lines online (with out any shutdown).
There is company TD Williomsons who provides STOPPLING services.
For more clarity you can see the below link or visit
http://www.youtube.com/watch?v=9oIQk4fakaI ...

Very rightly said by Ashfaq Anwer.
I also recommend welding and then rolling of tubes based on above reasons.
"Heat Exchanger Design and Fabrication" Book by Stanly Yokell also recommends to do welding and then rolling.
In case of PWHT requirements, rolling should be performed by after welding and ...

You can perform PWHT of CS vessels (more than 37.5 mm thk) or piping of CS (more than 19.05 mm thk) with SS 304L or SS 316 L caldding.
SS cladding will not be sensitized because of low carbon contents.
There is a curve called "Time Temperature Sensitization Curve" which tells about the possibility ...