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 $53.91   8.165%   $58.3118   $58.31   $58.30   $58.30  
  Purchase #2 $141.53   5.509%   $149.3269   $149.33   $149.35   $149.30  
  Purchase #3 $216.64   4.576%   $226.5534   $226.55   $226.55   $226.60  
  Purchase #4 $104.93   7.298%   $112.5878   $112.59   $112.60   $112.60  
  Purchase #5 $111.27   9.168%   $121.4712   $121.47   $121.45   $121.50  
  Purchase #6 $12.78   3.520%   $13.2299   $13.23   $13.25   $13.20  
  Purchase #7 $206.84   6.659%   $220.6135   $220.61   $220.60   $220.60  
  7 purchase totals = $847.90       $902.0945   $902.09   $902.10   $902.10