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 $179.28   3.270%   $185.1425   $185.14   $185.15   $185.10  
  Purchase #2 $63.60   10.670%   $70.3861   $70.39   $70.40   $70.40  
  Purchase #3 $130.79   6.656%   $139.4954   $139.50   $139.50   $139.50  
  Purchase #4 $7.83   8.402%   $8.4879   $8.49   $8.50   $8.50  
  Purchase #5 $97.96   6.316%   $104.1472   $104.15   $104.15   $104.10  
  Purchase #6 $45.41   6.702%   $48.4534   $48.45   $48.45   $48.50  
  Purchase #7 $114.44   13.592%   $129.9947   $129.99   $130.00   $130.00  
  7 purchase totals = $639.31       $686.1072   $686.11   $686.15   $686.10