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 $37.86   6.814%   $40.4398   $40.44   $40.45   $40.40  
  Purchase #2 $83.85   4.310%   $87.4639   $87.46   $87.45   $87.50  
  Purchase #3 $163.94   11.788%   $183.2652   $183.27   $183.25   $183.30  
  Purchase #4 $17.88   13.605%   $20.3126   $20.31   $20.30   $20.30  
  Purchase #5 $237.65   6.149%   $252.2631   $252.26   $252.25   $252.30  
  Purchase #6 $28.57   11.293%   $31.7964   $31.80   $31.80   $31.80  
  Purchase #7 $208.72   7.353%   $224.0672   $224.07   $224.05   $224.10  
  7 purchase totals = $778.47       $839.6082   $839.61   $839.55   $839.70