Depends on the specialty. If you've worked with them when they were also an attending, then go for it. If it's a surgical specialty (or EM), their chief residents are still residents, so it's better to get a letter from someone else. If it's peds, their chief residents tend to function as junior faculty members (serving as attending on wards, etc), so that would be reasonable. If you can, I'd still suggest getting a letter from a more senior attending--I asked an attending who had been an attending less than a year for a letter, and she said she'd be happy to write it, but I would get a stronger letter if I asked the guy who had been practicing for 20+ years already.