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 $19.32   5.227%   $20.3299   $20.33   $20.35   $20.30  
  Purchase #2 $12.47   7.363%   $13.3882   $13.39   $13.40   $13.40  
  Purchase #3 $124.34   11.640%   $138.8132   $138.81   $138.80   $138.80  
  Purchase #4 $178.44   3.575%   $184.8192   $184.82   $184.80   $184.80  
  Purchase #5 $49.67   3.546%   $51.4313   $51.43   $51.45   $51.40  
  Purchase #6 $77.36   13.353%   $87.6899   $87.69   $87.70   $87.70  
  Purchase #7 $20.52   6.838%   $21.9232   $21.92   $21.90   $21.90  
  7 purchase totals = $482.12       $518.3949   $518.39   $518.40   $518.30