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 $242.78   12.550%   $273.2489   $273.25   $273.25   $273.20  
  Purchase #2 $59.67   9.432%   $65.2981   $65.30   $65.30   $65.30  
  Purchase #3 $89.65   4.339%   $93.5399   $93.54   $93.55   $93.50  
  Purchase #4 $125.30   10.523%   $138.4853   $138.49   $138.50   $138.50  
  Purchase #5 $166.90   6.755%   $178.1741   $178.17   $178.15   $178.20  
  Purchase #6 $207.52   5.462%   $218.8547   $218.85   $218.85   $218.90  
  Purchase #7 $89.66   9.713%   $98.3687   $98.37   $98.35   $98.40  
  7 purchase totals = $981.48       $1,065.9697   $1,065.97   $1,065.95   $1,066.00