If you are unsure if you will be able to get pre-approved or not for a mortgage, then it is a good idea to go through the pre-approval process right away, as it:
*Alerts you to any issues that prevent you from getting approved. Then you have time to fix those, before you actually want to buy a house.
*Gives you the information you need to know how much money you will need to save for your down payment and closing costs.
*Educates you on the price range that you actually feel comfortable spending, versus just running financial calculators on various websites.
*Allows you to meet a Loan Officer that will become your trusted advisor and help you through the process. If you don't like the first loan officer that you talk to, move on to another.