It's not like there's a size where additional caster becomes mandatory, it's just an effect of increasing the tire width. The wider the front tires get, the more they tend to track the imperfections in the road surface. I've run 275's with as little as about +3.5° caster and as much as +8° caster, for my car and my driving preferences I've found that +6.5° works best for me and my set up. But it's not like the car was dangerous to drive at +3.5° caster, it's just more pleasurable at +6.5° - feels more stable, fewer steering corrections needed etc. Above that the trade off between stability and steering effort isn't worth the additional steering effort, at least IMO, but I also run a 16:1 manual steering box. And it depends on your roads too, apparently a lot of the roads I drive have "ruts" from semi-truck traffic and at lower positive caster values the 275's like to seek them out and follow them. If all your roads are nice and smooth it wouldn't be as significant an occurrence.
I'd say that since the offset bushings should get you into the +3.5° to +4° caster range you shouldn't have any really noticeable effects if you stay with a front tire less than 245 wide. At 245 or above I'd personally want more than +4° of caster, which the offset UCA bushings and stock UCA's probably won't get you, especially with some rake on the car. Some folks have been able to get more than that but it will depend on your car and settings. Typically though if you want much more than +4° you'd need to start looking into tubular or adjustable UCA's.