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 $242.44   11.772%   $270.98   $270.98   $271.00   $271.00  
  Purchase #2 $136.51   12.787%   $153.9655   $153.97   $153.95   $154.00  
  Purchase #3 $161.57   9.516%   $176.945   $176.95   $176.95   $176.90  
  Purchase #4 $60.60   6.838%   $64.7438   $64.74   $64.75   $64.70  
  Purchase #5 $152.29   5.473%   $160.6248   $160.62   $160.60   $160.60  
  Purchase #6 $88.70   5.834%   $93.8748   $93.87   $93.85   $93.90  
  Purchase #7 $88.62   13.307%   $100.4127   $100.41   $100.40   $100.40  
  7 purchase totals = $930.73       $1,021.5466   $1,021.54   $1,021.50   $1,021.50