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 $138.86   13.561%   $157.6908   $157.69   $157.70   $157.70  
  Purchase #2 $223.88   12.833%   $252.6105   $252.61   $252.60   $252.60  
  Purchase #3 $143.44   12.667%   $161.6095   $161.61   $161.60   $161.60  
  Purchase #4 $121.69   10.203%   $134.106   $134.11   $134.10   $134.10  
  Purchase #5 $227.86   6.115%   $241.7936   $241.79   $241.80   $241.80  
  Purchase #6 $110.78   3.162%   $114.2829   $114.28   $114.30   $114.30  
  Purchase #7 $203.46   5.238%   $214.1172   $214.12   $214.10   $214.10  
  7 purchase totals = $1,169.97       $1,276.2105   $1,276.21   $1,276.20   $1,276.20