In 10 years of iron block sleeving and somewhere in the neighborhood of 5000 holes sleeved I can't say we have actually considered welding a thin wall iron block bore prior to sleeving.. ..considering the remaining wall can be as shallow as .035" some openings can show up and welding iron would mosy likely not be an option as it would do more damage than good....this is why using liquid nitrogen to install sleeves is so popular these days....little parent bore damage is done...cylinder bores with damage/cracks/fractures must certainly be checked...we use a general rule of 1" from the top or bottom as general guideline for a bore that can be sleeved and saved from damage.....a liquid sealant pulled into the block by vacuum and then baked in an oven is one way to save the blocks with potential leaks from a questionable sleeve...these images might show you some of the process...
http://www.gessford.com/images/FEPressurecheck.htm
We also repair aluminum blocks that are sleeved...this is a different process completely....these sleeves are generally a flanged sleeve with liner flange protrusion to help seal the gasket or o-rings.....welding a parent bore in an aluminum block that doesn't require liner press fit is different and more common....
Regards, george.