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 $18.53   9.266%   $20.247   $20.25   $20.25   $20.20  
  Purchase #2 $138.53   4.687%   $145.0229   $145.02   $145.00   $145.00  
  Purchase #3 $139.84   3.797%   $145.1497   $145.15   $145.15   $145.10  
  Purchase #4 $150.81   8.494%   $163.6198   $163.62   $163.60   $163.60  
  Purchase #5 $233.48   7.810%   $251.7148   $251.71   $251.70   $251.70  
  Purchase #6 $202.30   11.263%   $225.085   $225.09   $225.10   $225.10  
  Purchase #7 $34.41   12.529%   $38.7212   $38.72   $38.70   $38.70  
  7 purchase totals = $917.90       $989.5604   $989.56   $989.50   $989.40