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 $39.93   4.794%   $41.8442   $41.84   $41.85   $41.80  
  Purchase #2 $46.98   6.223%   $49.9036   $49.90   $49.90   $49.90  
  Purchase #3 $216.62   8.562%   $235.167   $235.17   $235.15   $235.20  
  Purchase #4 $233.31   11.194%   $259.4267   $259.43   $259.45   $259.40  
  Purchase #5 $77.34   10.698%   $85.6138   $85.61   $85.60   $85.60  
  Purchase #6 $239.25   11.779%   $267.4313   $267.43   $267.45   $267.40  
  Purchase #7 $246.95   13.788%   $280.9995   $281.00   $281.00   $281.00  
  7 purchase totals = $1,100.38       $1,220.3861   $1,220.38   $1,220.40   $1,220.30