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 $20.63   3.775%   $21.4088   $21.41   $21.40   $21.40  
  Purchase #2 $167.55   4.579%   $175.2221   $175.22   $175.20   $175.20  
  Purchase #3 $76.74   6.362%   $81.6222   $81.62   $81.60   $81.60  
  Purchase #4 $53.59   13.418%   $60.7807   $60.78   $60.80   $60.80  
  Purchase #5 $16.76   7.871%   $18.0792   $18.08   $18.10   $18.10  
  Purchase #6 $157.79   7.762%   $170.0377   $170.04   $170.05   $170.00  
  Purchase #7 $45.55   9.309%   $49.7902   $49.79   $49.80   $49.80  
  7 purchase totals = $538.61       $576.9409   $576.94   $576.95   $576.90