joshuajnoble:
I'm setting a timer interrupt on TIMER1, would this somehow affect the timing in my sketch (other than introducing another method to run at an interval)? ...Tiny core...
Are you using the Tiny Core?
http://code.google.com/p/arduino-tiny/
Or the core available from the MIT website?
http://hlt.media.mit.edu/?p=1695
https://github.com/damellis/attiny/zipball/Arduino1
By default, in the Tiny Core, timer 1 is used for millis (and its ilk). The MIT core always uses timer 0 for millis.
The Tiny Core has a compiler switch to use timer 0.