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 $20.29   6.807%   $21.6711   $21.67   $21.65   $21.70  
  Purchase #2 $23.61   11.745%   $26.383   $26.38   $26.40   $26.40  
  Purchase #3 $142.36   11.208%   $158.3157   $158.32   $158.30   $158.30  
  Purchase #4 $220.42   11.152%   $245.0012   $245.00   $245.00   $245.00  
  Purchase #5 $67.88   10.782%   $75.1988   $75.20   $75.20   $75.20  
  Purchase #6 $107.56   9.141%   $117.3921   $117.39   $117.40   $117.40  
  Purchase #7 $208.77   5.243%   $219.7158   $219.72   $219.70   $219.70  
  7 purchase totals = $790.89       $863.6777   $863.68   $863.65   $863.70