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 $29.99   5.640%   $31.6814   $31.68   $31.70   $31.70  
  Purchase #2 $56.46   13.787%   $64.2441   $64.24   $64.25   $64.20  
  Purchase #3 $172.83   3.630%   $179.1037   $179.10   $179.10   $179.10  
  Purchase #4 $54.50   7.234%   $58.4425   $58.44   $58.45   $58.40  
  Purchase #5 $179.52   4.688%   $187.9359   $187.94   $187.95   $187.90  
  Purchase #6 $212.44   6.839%   $226.9688   $226.97   $226.95   $227.00  
  Purchase #7 $201.65   11.168%   $224.1703   $224.17   $224.15   $224.20  
  7 purchase totals = $907.39       $972.5467   $972.54   $972.55   $972.50