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 $242.29   13.511%   $275.0258   $275.03   $275.05   $275.00  
  Purchase #2 $86.67   13.394%   $98.2786   $98.28   $98.30   $98.30  
  Purchase #3 $136.49   13.779%   $155.297   $155.30   $155.30   $155.30  
  Purchase #4 $53.67   4.775%   $56.2327   $56.23   $56.25   $56.20  
  Purchase #5 $244.54   5.548%   $258.1071   $258.11   $258.10   $258.10  
  Purchase #6 $136.83   12.652%   $154.1417   $154.14   $154.15   $154.10  
  Purchase #7 $205.26   4.818%   $215.1494   $215.15   $215.15   $215.10  
  7 purchase totals = $1,105.75       $1,212.2323   $1,212.24   $1,212.30   $1,212.10