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 $20.47   6.856%   $21.8734   $21.87   $21.85   $21.90  
  Purchase #2 $20.84   9.396%   $22.7981   $22.80   $22.80   $22.80  
  Purchase #3 $189.88   10.164%   $209.1794   $209.18   $209.20   $209.20  
  Purchase #4 $25.58   7.167%   $27.4133   $27.41   $27.40   $27.40  
  Purchase #5 $36.73   7.833%   $39.6071   $39.61   $39.60   $39.60  
  Purchase #6 $250.79   13.362%   $284.3006   $284.30   $284.30   $284.30  
  Purchase #7 $225.92   13.868%   $257.2506   $257.25   $257.25   $257.30  
  7 purchase totals = $770.21       $862.4225   $862.42   $862.40   $862.50