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 $182.58   12.411%   $205.24   $205.24   $205.25   $205.20  
  Purchase #2 $23.47   12.851%   $26.4861   $26.49   $26.50   $26.50  
  Purchase #3 $24.55   13.442%   $27.85   $27.85   $27.85   $27.90  
  Purchase #4 $86.46   13.863%   $98.4459   $98.45   $98.45   $98.40  
  Purchase #5 $138.97   4.701%   $145.503   $145.50   $145.50   $145.50  
  Purchase #6 $206.62   4.219%   $215.3373   $215.34   $215.35   $215.30  
  Purchase #7 $73.94   8.199%   $80.0023   $80.00   $80.00   $80.00  
  7 purchase totals = $736.59       $798.8646   $798.87   $798.90   $798.80