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 $20.55   4.644%   $21.5043   $21.50   $21.50   $21.50  
  Purchase #2 $132.88   10.786%   $147.2124   $147.21   $147.20   $147.20  
  Purchase #3 $125.80   7.267%   $134.9419   $134.94   $134.95   $134.90  
  Purchase #4 $76.97   4.784%   $80.6522   $80.65   $80.65   $80.70  
  Purchase #5 $126.26   6.787%   $134.8293   $134.83   $134.85   $134.80  
  Purchase #6 $86.40   7.467%   $92.8515   $92.85   $92.85   $92.90  
  Purchase #7 $50.58   7.652%   $54.4504   $54.45   $54.45   $54.50  
  7 purchase totals = $619.44       $666.4420   $666.43   $666.45   $666.50