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 $16.67   9.695%   $18.2862   $18.29   $18.30   $18.30  
  Purchase #2 $62.31   8.129%   $67.3752   $67.38   $67.40   $67.40  
  Purchase #3 $40.69   9.390%   $44.5108   $44.51   $44.50   $44.50  
  Purchase #4 $150.62   13.734%   $171.3062   $171.31   $171.30   $171.30  
  Purchase #5 $143.36   9.130%   $156.4488   $156.45   $156.45   $156.40  
  Purchase #6 $14.74   4.560%   $15.4121   $15.41   $15.40   $15.40  
  Purchase #7 $216.59   12.684%   $244.0623   $244.06   $244.05   $244.10  
  7 purchase totals = $644.98       $717.4016   $717.41   $717.40   $717.40