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 $5.48   6.186%   $5.819   $5.82   $5.80   $5.80  
  Purchase #2 $18.70   12.430%   $21.0244   $21.02   $21.00   $21.00  
  Purchase #3 $102.87   9.272%   $112.4081   $112.41   $112.40   $112.40  
  Purchase #4 $143.53   11.162%   $159.5508   $159.55   $159.55   $159.60  
  Purchase #5 $15.47   10.416%   $17.0814   $17.08   $17.10   $17.10  
  Purchase #6 $27.99   5.772%   $29.6056   $29.61   $29.60   $29.60  
  Purchase #7 $228.61   10.506%   $252.6278   $252.63   $252.65   $252.60  
  7 purchase totals = $542.65       $598.1171   $598.12   $598.10   $598.10