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 $37.61   6.341%   $39.9949   $39.99   $40.00   $40.00  
  Purchase #2 $56.38   6.297%   $59.9302   $59.93   $59.95   $59.90  
  Purchase #3 $204.65   9.529%   $224.1511   $224.15   $224.15   $224.20  
  Purchase #4 $48.36   5.451%   $50.9961   $51.00   $51.00   $51.00  
  Purchase #5 $222.30   7.745%   $239.5171   $239.52   $239.50   $239.50  
  Purchase #6 $51.38   6.766%   $54.8564   $54.86   $54.85   $54.90  
  Purchase #7 $172.37   10.259%   $190.0534   $190.05   $190.05   $190.10  
  7 purchase totals = $793.05       $859.4992   $859.50   $859.50   $859.60