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 $111.30   11.705%   $124.3277   $124.33   $124.35   $124.30  
  Purchase #2 $194.71   4.212%   $202.9112   $202.91   $202.90   $202.90  
  Purchase #3 $231.94   5.318%   $244.2746   $244.27   $244.25   $244.30  
  Purchase #4 $48.99   8.285%   $53.0488   $53.05   $53.05   $53.00  
  Purchase #5 $18.30   5.731%   $19.3488   $19.35   $19.35   $19.30  
  Purchase #6 $81.33   6.545%   $86.653   $86.65   $86.65   $86.70  
  Purchase #7 $76.97   10.870%   $85.3366   $85.34   $85.35   $85.30  
  7 purchase totals = $763.54       $815.9007   $815.90   $815.90   $815.80