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 $239.92   11.817%   $268.2713   $268.27   $268.25   $268.30  
  Purchase #2 $124.82   3.529%   $129.2249   $129.22   $129.20   $129.20  
  Purchase #3 $201.63   11.575%   $224.9687   $224.97   $224.95   $225.00  
  Purchase #4 $35.25   11.459%   $39.2893   $39.29   $39.30   $39.30  
  Purchase #5 $65.67   7.447%   $70.5604   $70.56   $70.55   $70.60  
  Purchase #6 $5.28   9.867%   $5.801   $5.80   $5.80   $5.80  
  Purchase #7 $69.53   8.266%   $75.2773   $75.28   $75.30   $75.30  
  7 purchase totals = $742.10       $813.3929   $813.39   $813.35   $813.50