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 $243.83   12.276%   $273.7626   $273.76   $273.75   $273.80  
  Purchase #2 $37.31   5.455%   $39.3453   $39.35   $39.35   $39.30  
  Purchase #3 $100.97   12.628%   $113.7205   $113.72   $113.70   $113.70  
  Purchase #4 $114.75   10.542%   $126.8469   $126.85   $126.85   $126.80  
  Purchase #5 $203.43   5.181%   $213.9697   $213.97   $213.95   $214.00  
  Purchase #6 $241.75   6.226%   $256.8014   $256.80   $256.80   $256.80  
  Purchase #7 $52.47   5.308%   $55.2551   $55.26   $55.25   $55.30  
  7 purchase totals = $994.51       $1,079.7015   $1,079.71   $1,079.65   $1,079.70