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 $14.61   8.841%   $15.9017   $15.90   $15.90   $15.90  
  Purchase #2 $9.79   10.256%   $10.7941   $10.79   $10.80   $10.80  
  Purchase #3 $112.42   9.346%   $122.9268   $122.93   $122.95   $122.90  
  Purchase #4 $138.51   12.289%   $155.5315   $155.53   $155.55   $155.50  
  Purchase #5 $75.95   11.770%   $84.8893   $84.89   $84.90   $84.90  
  Purchase #6 $29.47   13.691%   $33.5047   $33.50   $33.50   $33.50  
  Purchase #7 $127.81   7.272%   $137.1043   $137.10   $137.10   $137.10  
  7 purchase totals = $508.56       $560.6524   $560.64   $560.70   $560.60