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 $9.35   7.739%   $10.0736   $10.07   $10.05   $10.10  
  Purchase #2 $18.45   6.855%   $19.7147   $19.71   $19.70   $19.70  
  Purchase #3 $187.42   6.146%   $198.9388   $198.94   $198.95   $198.90  
  Purchase #4 $5.67   13.203%   $6.4186   $6.42   $6.40   $6.40  
  Purchase #5 $14.47   12.517%   $16.2812   $16.28   $16.30   $16.30  
  Purchase #6 $124.76   5.419%   $131.5207   $131.52   $131.50   $131.50  
  Purchase #7 $182.42   11.868%   $204.0696   $204.07   $204.05   $204.10  
  7 purchase totals = $542.54       $587.0172   $587.01   $586.95   $587.00