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 $20.99   3.123%   $21.6455   $21.65   $21.65   $21.60  
  Purchase #2 $152.70   10.668%   $168.99   $168.99   $169.00   $169.00  
  Purchase #3 $168.27   7.392%   $180.7085   $180.71   $180.70   $180.70  
  Purchase #4 $38.36   8.833%   $41.7483   $41.75   $41.75   $41.70  
  Purchase #5 $15.32   4.303%   $15.9792   $15.98   $16.00   $16.00  
  Purchase #6 $248.72   6.419%   $264.6853   $264.69   $264.70   $264.70  
  Purchase #7 $197.75   11.726%   $220.9382   $220.94   $220.95   $220.90  
  7 purchase totals = $842.11       $914.6950   $914.71   $914.75   $914.60