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 $138.68   6.865%   $148.2004   $148.20   $148.20   $148.20  
  Purchase #2 $236.66   8.470%   $256.7051   $256.71   $256.70   $256.70  
  Purchase #3 $182.72   7.257%   $195.98   $195.98   $196.00   $196.00  
  Purchase #4 $205.43   12.752%   $231.6264   $231.63   $231.65   $231.60  
  Purchase #5 $7.87   5.414%   $8.2961   $8.30   $8.30   $8.30  
  Purchase #6 $62.26   3.759%   $64.6004   $64.60   $64.60   $64.60  
  Purchase #7 $109.65   5.525%   $115.7082   $115.71   $115.70   $115.70  
  7 purchase totals = $943.27       $1,021.1166   $1,021.13   $1,021.15   $1,021.10