If, in the case you require the accurate 38khz for generating a carrier for infra-red you can be a little lax with the accuracy as most receivers can cope with some variation. Rather than use a timer, one method is to use code delays. Another method is to use some hardware like a 555 timer ic to generate the carrier and have a port pin gate it on and off. The bonus of using a 555 timer ic is that it has good current drive for the infrared led.

As usual it often pays to tell us WHY you want to achieve a certain outcome so we can better understand the whole scope of the problem.