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 $19.59   9.432%   $21.4377   $21.44   $21.45   $21.40  
  Purchase #2 $182.52   13.478%   $207.12   $207.12   $207.10   $207.10  
  Purchase #3 $203.47   9.240%   $222.2706   $222.27   $222.25   $222.30  
  Purchase #4 $118.56   10.373%   $130.8582   $130.86   $130.85   $130.90  
  Purchase #5 $83.79   10.143%   $92.2888   $92.29   $92.30   $92.30  
  Purchase #6 $53.42   6.691%   $56.9943   $56.99   $57.00   $57.00  
  Purchase #7 $124.35   3.360%   $128.5282   $128.53   $128.55   $128.50  
  7 purchase totals = $785.70       $859.4978   $859.50   $859.50   $859.50