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 $54.29   6.873%   $58.0214   $58.02   $58.00   $58.00  
  Purchase #2 $174.29   13.412%   $197.6658   $197.67   $197.65   $197.70  
  Purchase #3 $244.81   9.386%   $267.7879   $267.79   $267.80   $267.80  
  Purchase #4 $50.33   3.697%   $52.1907   $52.19   $52.20   $52.20  
  Purchase #5 $141.43   6.325%   $150.3754   $150.38   $150.40   $150.40  
  Purchase #6 $53.48   9.681%   $58.6574   $58.66   $58.65   $58.70  
  Purchase #7 $50.37   11.315%   $56.0694   $56.07   $56.05   $56.10  
  7 purchase totals = $769.00       $840.7680   $840.78   $840.75   $840.90