Another reason you need to have a clue about your down payment is because it affects how you write your offer to purchase a home. Not only are you required to put your down payment information in the offer, but different loan programs have different rules which also affect how you write your offer. This is especially important when dealing with FHA and VA loans.
If you are asking the seller to pay all or part of your closing costs, you have to be certain your loan program allows what you are asking. For smaller down payments, lenders allow the seller to pay less closing costs than for larger down payments. Some loan programs will allow a seller to pay certain types of costs, but not others.
Finally, your down payment also affects your ability to qualify for a loan. When you make a small down payment, lenders are fairly strict about having you conform to their underwriting guidelines. For larger down payments, they will tend to make allowances or exceptions to the rules.
