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 $59.75   11.255%   $66.4749   $66.47   $66.45   $66.50  
  Purchase #2 $15.43   12.693%   $17.3885   $17.39   $17.40   $17.40  
  Purchase #3 $219.34   11.415%   $244.3777   $244.38   $244.40   $244.40  
  Purchase #4 $224.83   9.574%   $246.3552   $246.36   $246.35   $246.40  
  Purchase #5 $168.75   12.304%   $189.513   $189.51   $189.50   $189.50  
  Purchase #6 $8.97   13.170%   $10.1513   $10.15   $10.15   $10.20  
  Purchase #7 $42.56   11.130%   $47.2969   $47.30   $47.30   $47.30  
  7 purchase totals = $739.63       $821.5575   $821.56   $821.55   $821.70