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 $246.94   10.850%   $273.733   $273.73   $273.75   $273.70  
  Purchase #2 $18.34   12.323%   $20.6   $20.60   $20.60   $20.60  
  Purchase #3 $154.63   5.816%   $163.6233   $163.62   $163.60   $163.60  
  Purchase #4 $86.29   6.748%   $92.1128   $92.11   $92.10   $92.10  
  Purchase #5 $149.46   4.658%   $156.4218   $156.42   $156.40   $156.40  
  Purchase #6 $33.72   9.783%   $37.0188   $37.02   $37.00   $37.00  
  Purchase #7 $150.63   10.465%   $166.3934   $166.39   $166.40   $166.40  
  7 purchase totals = $840.01       $909.9031   $909.89   $909.85   $909.80