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 $21.56   13.616%   $24.4956   $24.50   $24.50   $24.50  
  Purchase #2 $148.99   9.179%   $162.6658   $162.67   $162.65   $162.70  
  Purchase #3 $48.81   13.687%   $55.4906   $55.49   $55.50   $55.50  
  Purchase #4 $10.41   13.795%   $11.8461   $11.85   $11.85   $11.80  
  Purchase #5 $156.91   8.680%   $170.5298   $170.53   $170.55   $170.50  
  Purchase #6 $135.78   11.191%   $150.9751   $150.98   $151.00   $151.00  
  Purchase #7 $91.65   5.844%   $97.006   $97.01   $97.00   $97.00  
  7 purchase totals = $614.11       $673.0090   $673.03   $673.05   $673.00