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 $18.99   12.871%   $21.4342   $21.43   $21.45   $21.40  
  Purchase #2 $239.56   10.263%   $264.146   $264.15   $264.15   $264.10  
  Purchase #3 $83.46   3.441%   $86.3319   $86.33   $86.35   $86.30  
  Purchase #4 $37.61   8.383%   $40.7628   $40.76   $40.75   $40.80  
  Purchase #5 $177.25   13.834%   $201.7708   $201.77   $201.75   $201.80  
  Purchase #6 $197.37   12.810%   $222.6531   $222.65   $222.65   $222.70  
  Purchase #7 $32.46   13.289%   $36.7736   $36.77   $36.75   $36.80  
  7 purchase totals = $786.70       $873.8724   $873.86   $873.85   $873.90