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 $52.87   11.824%   $59.1213   $59.12   $59.10   $59.10  
  Purchase #2 $60.29   7.236%   $64.6526   $64.65   $64.65   $64.70  
  Purchase #3 $152.32   11.616%   $170.0135   $170.01   $170.00   $170.00  
  Purchase #4 $25.71   3.386%   $26.5805   $26.58   $26.60   $26.60  
  Purchase #5 $13.73   13.477%   $15.5804   $15.58   $15.60   $15.60  
  Purchase #6 $235.97   4.615%   $246.86   $246.86   $246.85   $246.90  
  Purchase #7 $50.49   12.641%   $56.8724   $56.87   $56.85   $56.90  
  7 purchase totals = $591.38       $639.6807   $639.67   $639.65   $639.80