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 $20.92   9.521%   $22.9118   $22.91   $22.90   $22.90  
  Purchase #2 $21.48   8.688%   $23.3462   $23.35   $23.35   $23.30  
  Purchase #3 $174.52   9.276%   $190.7085   $190.71   $190.70   $190.70  
  Purchase #4 $89.36   4.565%   $93.4393   $93.44   $93.45   $93.40  
  Purchase #5 $129.75   10.525%   $143.4062   $143.41   $143.40   $143.40  
  Purchase #6 $180.38   13.363%   $204.4842   $204.48   $204.50   $204.50  
  Purchase #7 $39.99   5.611%   $42.2338   $42.23   $42.25   $42.20  
  7 purchase totals = $656.40       $720.5300   $720.53   $720.55   $720.40