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 $25.57   9.792%   $28.0738   $28.07   $28.05   $28.10  
  Purchase #2 $189.95   5.350%   $200.1123   $200.11   $200.10   $200.10  
  Purchase #3 $35.74   11.160%   $39.7286   $39.73   $39.75   $39.70  
  Purchase #4 $14.56   4.113%   $15.1589   $15.16   $15.15   $15.20  
  Purchase #5 $38.26   7.813%   $41.2493   $41.25   $41.25   $41.20  
  Purchase #6 $147.64   11.103%   $164.0325   $164.03   $164.05   $164.00  
  Purchase #7 $140.43   12.195%   $157.5554   $157.56   $157.55   $157.60  
  7 purchase totals = $592.15       $645.9108   $645.91   $645.90   $645.90