Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $236.65   13.686%   $269.0379   $269.04   $269.05   $269.00  
  Purchase #2 $163.75   7.384%   $175.8413   $175.84   $175.85   $175.80  
  Purchase #3 $137.94   8.230%   $149.2925   $149.29   $149.30   $149.30  
  Purchase #4 $49.68   7.255%   $53.2843   $53.28   $53.30   $53.30  
  Purchase #5 $67.70   13.310%   $76.7109   $76.71   $76.70   $76.70  
  Purchase #6 $139.71   13.441%   $158.4884   $158.49   $158.50   $158.50  
  Purchase #7 $84.85   12.374%   $95.3493   $95.35   $95.35   $95.30  
  7 purchase totals = $880.28       $978.0046   $978.00   $978.05   $977.90