Having been intimately involved in in-the-field column splices and moment connections, backing bars are a lot of trouble and make the item hard to fit up. What works best IMHO is a groove-weld prep with a 'meaty' land. This land is to be fit tight to the mating member - easy for the ironworkers, as they just swing in the piece and tack it on the land. Then when the groove has a root and hot-pass, they put a fat fillet on the back side. Member stays straight. The finished weld is a partial-penn groove weld with fillet reinforcement on BOTH sides. NOTE: All decent welders will add a reinforcing fillet to groove welds in tee-joints, like moment connections between beams and columns. That reentrant corner that a weld-cap forms at the column is very bad for stress-concentration, and it naturally 'calls to the welder' to be faired in to eliminate it.
If you are worried about fatigue problems due to the partial-penn, this detail can be changed to full-penn by backgouging the land-side of groove in the field. In this case, detail the bevels to be on the INSIDE of the beam. This allows the iron-heads to easily skim-gouge all the way across the beam, and not have to 'fight' the web. Pet peeve of mine - moment connections are usually detailed to allow the easy cuts to be made in the shop, then only a small 'rat hole' weld access cope is shop-made. Takes a good hour of work in the field just to get the rat-hole recut to allow the backing bar to be 'sort of' fit. Then when that full-penn moment made that traditional way is UT'd, the tech sees massive indications at the root, all due to a less-than-perfect fit of the backing bar in the web area. To add further insult to injury, the precut bars that the iron-heads get given are the exact length as the width of the beam's flanges. This leaves the poor welder with no run-out tab so he can make the full-penn weld full thickness at the ends of the flange.
All of the above means that even if full-penn, vs. partial-penn with fillets both sides, is needed, it is faster [thus cheaper] to backgouge. And if you cannot bring yourself to detail a backgouge, at least make the SHOP cope the web so that that d*amnable bar can be tacked in place without further work while hanging up in the air trying to get the beam welded.
