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 $25.77   3.130%   $26.5766   $26.58   $26.60   $26.60  
  Purchase #2 $205.90   8.499%   $223.3994   $223.40   $223.40   $223.40  
  Purchase #3 $118.75   13.507%   $134.7896   $134.79   $134.80   $134.80  
  Purchase #4 $169.66   6.381%   $180.486   $180.49   $180.50   $180.50  
  Purchase #5 $6.39   3.523%   $6.6151   $6.62   $6.60   $6.60  
  Purchase #6 $246.63   3.850%   $256.1253   $256.13   $256.15   $256.10  
  Purchase #7 $150.91   7.802%   $162.684   $162.68   $162.70   $162.70  
  7 purchase totals = $924.01       $990.6760   $990.69   $990.75   $990.70