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 $27.32   12.325%   $30.6872   $30.69   $30.70   $30.70  
  Purchase #2 $130.49   8.360%   $141.399   $141.40   $141.40   $141.40  
  Purchase #3 $158.58   13.702%   $180.3086   $180.31   $180.30   $180.30  
  Purchase #4 $198.58   8.365%   $215.1912   $215.19   $215.20   $215.20  
  Purchase #5 $104.77   13.362%   $118.7694   $118.77   $118.75   $118.80  
  Purchase #6 $104.70   3.634%   $108.5048   $108.50   $108.50   $108.50  
  Purchase #7 $7.89   11.258%   $8.7783   $8.78   $8.80   $8.80  
  7 purchase totals = $732.33       $803.6385   $803.64   $803.65   $803.70