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 $17.31   4.195%   $18.0362   $18.04   $18.05   $18.00  
  Purchase #2 $77.66   3.446%   $80.3362   $80.34   $80.35   $80.30  
  Purchase #3 $53.61   6.799%   $57.2549   $57.25   $57.25   $57.30  
  Purchase #4 $166.73   3.733%   $172.954   $172.95   $172.95   $173.00  
  Purchase #5 $61.52   3.374%   $63.5957   $63.60   $63.60   $63.60  
  Purchase #6 $217.26   6.584%   $231.5644   $231.56   $231.55   $231.60  
  Purchase #7 $44.66   9.117%   $48.7317   $48.73   $48.75   $48.70  
  7 purchase totals = $638.75       $672.4731   $672.47   $672.50   $672.50