I don't use a straight average rate, so yes, it will change based on the job.
For example, I have 5 separate pay rate classes for bidding purposes; top tier j-man down to helper. This is excluding any PM/Superintendent rates.
A job that is going to be mostly running pipe & pulling wire is going to take more of the higher wage labor pool than a job that is mostly digging ditches and dropping PVC in the ground. I divide up the labor hours for each phase of the job amongst the different labor wage pools that I will need to staff the job for each particular phase.
For small jobs, this isn't going to matter much, but for jobs that are a couple thousand hours and up, it can really affect your price. Not all jobs are going to use someone from each labor class either. The small stuff where its a 3-4 week project with 2-3 guys, only their labor wage class will be used in bidding. Well, that and the hours that are moved to the pre-fab shop for making their assemblies here in-house.