Hello! I am working in the audio postproduction of a movie and this is my first time working in Sonar 3Xe. I load the video and go to clock and I put the time code so it can sync with the video. My question is ¿if I change the Tempo will I loose sync in my dialogs and other fxs? ¿will the tempo changes alter the pitch of my sounds? ¿In what tempo should I work on?
It’s my first job and I don’t want to screw it up
 
Thank you for your help!!

You cannot edit a Broadcast WAV file's time stamp, however, you can revert a clip to its original time stamp. So for example, assume you create a BWF at 1:30 seconds with a sound effect and place it on the time line. If you change tempo, the clip will follow the M:B:T timeline and move to a different position. But if you revert to the original time stamp, it will re-position itself to start at 1:30.

It's possible to lose sync if the tempo changes in the default set up, but if you
1) CTRL-A to select all your clip,
2) Click on the "Clips" Tab in the Inspector,
3) choose "Absolute" for the "Time Base" ("musical" is the default),
the clips won't move if the tempo changes. Each individual clip can be either musical or Absolute, so if you need them to follow the tempo, they can.
If I'm not scoring, I generally leave the tempo at the default: but if you are using tempo-based effects, it is simple to do so. Note that whenever you insert/ record/ create a new clip, the Time Base will be "musical" so it will move if you change the tempo before the clip. 

I think I understand, if a create a wav file with fx export it and then import it to my time line and the tempo is different it will not sync? Sorry for being slow but i am just learning all these things
¿If I am creating a session from cero for audio postproduction what tempo should a work on?
 
Thank you very very much for your help I am kind of worried :)

Thank You OscarLaun!
¿So in my case that a don’t do scores I will always have to change the Time Base to Absolute? And simply let the tempo alone and don’t fiddle with it.
 
Thank you guys very very much, you are a life saver

Yes, that's what I do. One annoying thing is skipping to a certain time (pressing G), since you can't use the Skip To function to go to SMPTE, as far as I can tell.. But at 120 beats per minute, you get 30 measures + 1 per minute (as you start at measure 1), so it's a simple multiplication. e.g.: if you want to go to the 15 minute mark, multiply 15 by 30 and add 1 (451). 
Good luck!