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 $100.46   5.613%   $106.0988   $106.10   $106.10   $106.10  
  Purchase #2 $124.32   13.849%   $141.5371   $141.54   $141.55   $141.50  
  Purchase #3 $128.76   4.148%   $134.101   $134.10   $134.10   $134.10  
  Purchase #4 $9.41   6.213%   $9.9946   $9.99   $10.00   $10.00  
  Purchase #5 $120.43   8.551%   $130.728   $130.73   $130.75   $130.70  
  Purchase #6 $79.85   10.536%   $88.263   $88.26   $88.25   $88.30  
  Purchase #7 $28.53   10.765%   $31.6013   $31.60   $31.60   $31.60  
  7 purchase totals = $591.76       $642.3238   $642.32   $642.35   $642.30