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 $107.34   9.316%   $117.3398   $117.34   $117.35   $117.30  
  Purchase #2 $216.82   11.579%   $241.9256   $241.93   $241.95   $241.90  
  Purchase #3 $79.82   7.733%   $85.9925   $85.99   $86.00   $86.00  
  Purchase #4 $33.48   3.874%   $34.777   $34.78   $34.80   $34.80  
  Purchase #5 $41.84   12.153%   $46.9248   $46.92   $46.90   $46.90  
  Purchase #6 $244.55   4.530%   $255.6281   $255.63   $255.65   $255.60  
  Purchase #7 $50.65   11.783%   $56.6181   $56.62   $56.60   $56.60  
  7 purchase totals = $774.50       $839.2059   $839.21   $839.25   $839.10