It is always recommended to have a pre-qualification done prior to starting your home search, these are usually good for 90 days. Prior to having a lender start a pre-qualification and review credit, narrow down you search to the lender you feel is the best fit for you. It is not typically necessary to have several lenders reviewing your credit. You will need to address any recent inquiries on your credit report with the lender you decide to go with.
There is a lot to the process these days but this is one area that you can definitely help yourself with. In most cases derogatory credit situations rarely are the fault of someone other than ourselves. Be aware, be prudent, and you will be prepared!