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 $159.62   6.843%   $170.5428   $170.54   $170.55   $170.50  
  Purchase #2 $211.70   12.211%   $237.5507   $237.55   $237.55   $237.60  
  Purchase #3 $119.68   6.295%   $127.2139   $127.21   $127.20   $127.20  
  Purchase #4 $243.37   13.313%   $275.7698   $275.77   $275.75   $275.80  
  Purchase #5 $101.40   10.732%   $112.2822   $112.28   $112.30   $112.30  
  Purchase #6 $14.45   6.700%   $15.4182   $15.42   $15.40   $15.40  
  Purchase #7 $55.64   5.712%   $58.8182   $58.82   $58.80   $58.80  
  7 purchase totals = $905.86       $997.5958   $997.59   $997.55   $997.60