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 $246.52   12.676%   $277.7689   $277.77   $277.75   $277.80  
  Purchase #2 $102.59   9.297%   $112.1278   $112.13   $112.15   $112.10  
  Purchase #3 $189.52   13.311%   $214.747   $214.75   $214.75   $214.70  
  Purchase #4 $246.73   4.368%   $257.5072   $257.51   $257.50   $257.50  
  Purchase #5 $187.98   6.109%   $199.4637   $199.46   $199.45   $199.50  
  Purchase #6 $229.89   8.436%   $249.2835   $249.28   $249.30   $249.30  
  Purchase #7 $55.72   8.771%   $60.6072   $60.61   $60.60   $60.60  
  7 purchase totals = $1,258.95       $1,371.5053   $1,371.51   $1,371.50   $1,371.50