Curved canals are no problem once you get them "probed". The problem is getting to the apex in the first place, but that is about a half day lecture in itself.
Once the canals are probed then it is possible to circumferentially file them using the the new Fine-Cut Endo Contra-Angle and Fine-Cut Files. Genereally the largest file we need to use is a #30 so flexibility is not a problem.
In fact many canals can be cleansed and shaped with only "two or three files".
Filing curved canals only gets to be a problem if you have to use gutta percha and pack it around a curve, near the apex. Then the canal must not only be clean but ROUND as well. (Cement techniques only require that the canal be large enough to get a 30 gauge needle to place near the apex.) In order to get pluggers close to the apex, canal size must be much greater than it takes to get a canal clean.
With cement filling, canals can be kept much smaller and if they are a little out of round, no problem.
If you insist on using gutta percha we recommend finishing the canal with one or two LightSpeed files to make a seat for the gutta percha or as an alternative prepare the canal circumferentially first then dress the apical portion with a suitably sized rotary NiTi file.
If a canal is drastically curved, it is only necessary to pre-curve one or two Fine-Cut SPeed files, position them in place and start the handpiece moving it circumferentially. As long as the file is not used at or beyond the apex, there is no likelihood of zipping or stripping.