You need to make sure your plans are complete as possible before you start sending them out to get the bids. 

If you send out incomplete plans the sub-contractors will know you are new to the game and they will increase their bids accordingly. Meaning they know you are either going to be bugging them all the time, you may be impossible to please or you do not understand what Industry standards or build by code means. 

What that means for them is they will be spending extra time teaching you these things and they need to be paid for the extra time. That is fair. remember they also work very hard to provide for their own families. 

Make sure the plans are correct before sending them out. If you change the plans, then make sure they know so they can update their bids accordingly. If you make changes and do not tell them before they show up to do the work, do not be surprised if they hit you with a very big price change even for a small change on the plans. This is because it could affect three to five trips for them to do the additional work. 

Let them know as soon as you are even thinking about making changes. They may have a better solution to getting what you need for less money. 

Steve