I did all my own boxes up to the late 40s by single training. I can't comment on going any further than that, although I believe the lock and trap difficulties -- especially traps -- would start to get out of reach on the other side of 50th if you're just singling. A cautious approach and judicious use of things like loresinging and 416 will make you a very good locksmith in the first half of the game though.

Dave, Brandain's Bard