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 $84.80   10.169%   $93.4233   $93.42   $93.40   $93.40  
  Purchase #2 $19.42   12.315%   $21.8116   $21.81   $21.80   $21.80  
  Purchase #3 $46.35   7.165%   $49.671   $49.67   $49.65   $49.70  
  Purchase #4 $88.25   5.784%   $93.3544   $93.35   $93.35   $93.40  
  Purchase #5 $216.62   8.330%   $234.6644   $234.66   $234.65   $234.70  
  Purchase #6 $60.41   10.280%   $66.6201   $66.62   $66.60   $66.60  
  Purchase #7 $36.90   4.537%   $38.5742   $38.57   $38.55   $38.60  
  7 purchase totals = $552.75       $598.1190   $598.10   $598.00   $598.20