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 $122.35   13.436%   $138.7889   $138.79   $138.80   $138.80  
  Purchase #2 $91.55   3.127%   $94.4128   $94.41   $94.40   $94.40  
  Purchase #3 $6.95   6.447%   $7.3981   $7.40   $7.40   $7.40  
  Purchase #4 $106.81   3.118%   $110.1403   $110.14   $110.15   $110.10  
  Purchase #5 $98.57   3.574%   $102.0929   $102.09   $102.10   $102.10  
  Purchase #6 $16.73   13.182%   $18.9353   $18.94   $18.95   $18.90  
  Purchase #7 $134.26   8.666%   $145.895   $145.90   $145.90   $145.90  
  7 purchase totals = $577.22       $617.6633   $617.67   $617.70   $617.60