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 $18.75   8.425%   $20.3297   $20.33   $20.35   $20.30  
  Purchase #2 $81.78   7.525%   $87.9339   $87.93   $87.95   $87.90  
  Purchase #3 $133.72   9.513%   $146.4408   $146.44   $146.45   $146.40  
  Purchase #4 $153.53   13.599%   $174.4085   $174.41   $174.40   $174.40  
  Purchase #5 $6.33   10.620%   $7.0022   $7.00   $7.00   $7.00  
  Purchase #6 $225.37   13.142%   $254.9881   $254.99   $255.00   $255.00  
  Purchase #7 $152.48   10.657%   $168.7298   $168.73   $168.75   $168.70  
  7 purchase totals = $771.96       $859.8330   $859.83   $859.90   $859.70