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 $61.90   4.677%   $64.7951   $64.80   $64.80   $64.80  
  Purchase #2 $113.66   11.816%   $127.0901   $127.09   $127.10   $127.10  
  Purchase #3 $226.99   11.274%   $252.5809   $252.58   $252.60   $252.60  
  Purchase #4 $249.60   10.282%   $275.2639   $275.26   $275.25   $275.30  
  Purchase #5 $70.78   6.249%   $75.203   $75.20   $75.20   $75.20  
  Purchase #6 $23.50   6.596%   $25.0501   $25.05   $25.05   $25.10  
  Purchase #7 $145.86   9.479%   $159.6861   $159.69   $159.70   $159.70  
  7 purchase totals = $892.29       $979.6692   $979.67   $979.70   $979.80