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 $239.46   6.256%   $254.4406   $254.44   $254.45   $254.40  
  Purchase #2 $77.96   12.404%   $87.6302   $87.63   $87.65   $87.60  
  Purchase #3 $235.29   4.467%   $245.8004   $245.80   $245.80   $245.80  
  Purchase #4 $61.87   11.385%   $68.9139   $68.91   $68.90   $68.90  
  Purchase #5 $230.32   5.473%   $242.9254   $242.93   $242.95   $242.90  
  Purchase #6 $240.88   5.811%   $254.8775   $254.88   $254.90   $254.90  
  Purchase #7 $14.30   9.128%   $15.6053   $15.61   $15.60   $15.60  
  7 purchase totals = $1,100.08       $1,170.1933   $1,170.20   $1,170.25   $1,170.10