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 $18.38   9.300%   $20.0893   $20.09   $20.10   $20.10  
  Purchase #2 $12.90   6.547%   $13.7446   $13.74   $13.75   $13.70  
  Purchase #3 $243.86   3.193%   $251.6464   $251.65   $251.65   $251.60  
  Purchase #4 $149.69   7.824%   $161.4017   $161.40   $161.40   $161.40  
  Purchase #5 $61.99   10.570%   $68.5423   $68.54   $68.55   $68.50  
  Purchase #6 $225.90   5.743%   $238.8734   $238.87   $238.85   $238.90  
  Purchase #7 $211.47   4.376%   $220.7239   $220.72   $220.70   $220.70  
  7 purchase totals = $924.19       $975.0216   $975.01   $975.00   $974.90