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 $59.95   10.638%   $66.3275   $66.33   $66.35   $66.30  
  Purchase #2 $75.64   13.734%   $86.0284   $86.03   $86.05   $86.00  
  Purchase #3 $135.89   7.299%   $145.8086   $145.81   $145.80   $145.80  
  Purchase #4 $156.62   9.241%   $171.0933   $171.09   $171.10   $171.10  
  Purchase #5 $105.87   11.195%   $117.7221   $117.72   $117.70   $117.70  
  Purchase #6 $182.60   13.757%   $207.7203   $207.72   $207.70   $207.70  
  Purchase #7 $111.57   6.743%   $119.0932   $119.09   $119.10   $119.10  
  7 purchase totals = $828.14       $913.7934   $913.79   $913.80   $913.70