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 $55.26   6.606%   $58.9105   $58.91   $58.90   $58.90  
  Purchase #2 $75.62   13.674%   $85.9603   $85.96   $85.95   $86.00  
  Purchase #3 $239.36   7.133%   $256.4335   $256.43   $256.45   $256.40  
  Purchase #4 $63.29   4.167%   $65.9273   $65.93   $65.95   $65.90  
  Purchase #5 $23.29   4.293%   $24.2898   $24.29   $24.30   $24.30  
  Purchase #6 $53.91   3.812%   $55.965   $55.97   $55.95   $56.00  
  Purchase #7 $117.75   4.844%   $123.4538   $123.45   $123.45   $123.50  
  7 purchase totals = $628.48       $670.9402   $670.94   $670.95   $671.00