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 $21.78   5.740%   $23.0302   $23.03   $23.05   $23.00  
  Purchase #2 $121.36   6.765%   $129.57   $129.57   $129.55   $129.60  
  Purchase #3 $7.68   13.383%   $8.7078   $8.71   $8.70   $8.70  
  Purchase #4 $90.60   3.767%   $94.0129   $94.01   $94.00   $94.00  
  Purchase #5 $35.75   6.362%   $38.0244   $38.02   $38.00   $38.00  
  Purchase #6 $130.51   6.628%   $139.1602   $139.16   $139.15   $139.20  
  Purchase #7 $81.53   11.710%   $91.0772   $91.08   $91.10   $91.10  
  7 purchase totals = $489.21       $523.5827   $523.58   $523.55   $523.60