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 $86.90   4.714%   $90.9965   $91.00   $91.00   $91.00  
  Purchase #2 $238.59   7.840%   $257.2955   $257.30   $257.30   $257.30  
  Purchase #3 $60.30   8.823%   $65.6203   $65.62   $65.60   $65.60  
  Purchase #4 $71.79   9.451%   $78.5749   $78.57   $78.55   $78.60  
  Purchase #5 $247.71   12.523%   $278.7307   $278.73   $278.75   $278.70  
  Purchase #6 $8.67   5.784%   $9.1715   $9.17   $9.15   $9.20  
  Purchase #7 $132.78   13.416%   $150.5938   $150.59   $150.60   $150.60  
  7 purchase totals = $846.74       $930.9832   $930.98   $930.95   $931.00