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 $204.65   13.210%   $231.6843   $231.68   $231.70   $231.70  
  Purchase #2 $69.29   13.395%   $78.5714   $78.57   $78.55   $78.60  
  Purchase #3 $103.36   9.644%   $113.328   $113.33   $113.35   $113.30  
  Purchase #4 $163.42   3.621%   $169.3374   $169.34   $169.35   $169.30  
  Purchase #5 $20.57   13.842%   $23.4173   $23.42   $23.40   $23.40  
  Purchase #6 $216.81   10.819%   $240.2667   $240.27   $240.25   $240.30  
  Purchase #7 $42.97   9.792%   $47.1776   $47.18   $47.20   $47.20  
  7 purchase totals = $821.07       $903.7827   $903.79   $903.80   $903.80