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 $19.81   9.649%   $21.7215   $21.72   $21.70   $21.70  
  Purchase #2 $174.90   5.251%   $184.084   $184.08   $184.10   $184.10  
  Purchase #3 $18.79   7.870%   $20.2688   $20.27   $20.25   $20.30  
  Purchase #4 $42.88   3.526%   $44.3919   $44.39   $44.40   $44.40  
  Purchase #5 $65.72   13.460%   $74.5659   $74.57   $74.55   $74.60  
  Purchase #6 $130.82   7.151%   $140.1749   $140.17   $140.15   $140.20  
  Purchase #7 $60.36   11.136%   $67.0817   $67.08   $67.10   $67.10  
  7 purchase totals = $513.28       $552.2887   $552.28   $552.25   $552.40