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 $28.78   6.157%   $30.552   $30.55   $30.55   $30.60  
  Purchase #2 $29.81   8.760%   $32.4214   $32.42   $32.40   $32.40  
  Purchase #3 $55.61   11.750%   $62.1442   $62.14   $62.15   $62.10  
  Purchase #4 $103.42   6.683%   $110.3316   $110.33   $110.35   $110.30  
  Purchase #5 $133.87   3.568%   $138.6465   $138.65   $138.65   $138.60  
  Purchase #6 $53.52   3.242%   $55.2551   $55.26   $55.25   $55.30  
  Purchase #7 $102.34   9.503%   $112.0654   $112.07   $112.05   $112.10  
  7 purchase totals = $507.35       $541.4162   $541.42   $541.40   $541.40