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 $75.56   11.828%   $84.4972   $84.50   $84.50   $84.50  
  Purchase #2 $15.96   11.565%   $17.8058   $17.81   $17.80   $17.80  
  Purchase #3 $128.97   13.678%   $146.6105   $146.61   $146.60   $146.60  
  Purchase #4 $187.93   3.684%   $194.8533   $194.85   $194.85   $194.90  
  Purchase #5 $137.92   6.705%   $147.1675   $147.17   $147.15   $147.20  
  Purchase #6 $27.93   11.251%   $31.0724   $31.07   $31.05   $31.10  
  Purchase #7 $16.86   9.338%   $18.4344   $18.43   $18.45   $18.40  
  7 purchase totals = $591.13       $640.4411   $640.44   $640.40   $640.50