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 $50.44   11.868%   $56.4262   $56.43   $56.45   $56.40  
  Purchase #2 $61.92   13.227%   $70.1102   $70.11   $70.10   $70.10  
  Purchase #3 $124.54   3.376%   $128.7445   $128.74   $128.75   $128.70  
  Purchase #4 $151.47   9.282%   $165.5294   $165.53   $165.55   $165.50  
  Purchase #5 $199.81   11.759%   $223.3057   $223.31   $223.30   $223.30  
  Purchase #6 $44.44   6.209%   $47.1993   $47.20   $47.20   $47.20  
  Purchase #7 $76.99   3.616%   $79.774   $79.77   $79.75   $79.80  
  7 purchase totals = $709.61       $771.0893   $771.09   $771.10   $771.00