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 $24.34   8.306%   $26.3617   $26.36   $26.35   $26.40  
  Purchase #2 $193.87   6.683%   $206.8263   $206.83   $206.85   $206.80  
  Purchase #3 $67.29   4.631%   $70.4062   $70.41   $70.40   $70.40  
  Purchase #4 $236.33   6.862%   $252.547   $252.55   $252.55   $252.50  
  Purchase #5 $129.74   3.520%   $134.3068   $134.31   $134.30   $134.30  
  Purchase #6 $74.41   13.471%   $84.4338   $84.43   $84.45   $84.40  
  Purchase #7 $21.66   12.224%   $24.3077   $24.31   $24.30   $24.30  
  7 purchase totals = $747.64       $799.1895   $799.20   $799.20   $799.10