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 $19.71   3.264%   $20.3533   $20.35   $20.35   $20.40  
  Purchase #2 $160.59   9.245%   $175.4365   $175.44   $175.45   $175.40  
  Purchase #3 $88.25   13.120%   $99.8284   $99.83   $99.85   $99.80  
  Purchase #4 $91.67   6.724%   $97.8339   $97.83   $97.85   $97.80  
  Purchase #5 $94.98   3.664%   $98.4601   $98.46   $98.45   $98.50  
  Purchase #6 $110.88   13.185%   $125.4995   $125.50   $125.50   $125.50  
  Purchase #7 $83.76   11.103%   $93.0599   $93.06   $93.05   $93.10  
  7 purchase totals = $649.84       $710.4716   $710.47   $710.50   $710.50