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 $20.65   4.628%   $21.6057   $21.61   $21.60   $21.60  
  Purchase #2 $7.93   13.376%   $8.9907   $8.99   $9.00   $9.00  
  Purchase #3 $111.79   3.785%   $116.0213   $116.02   $116.00   $116.00  
  Purchase #4 $20.80   3.730%   $21.5758   $21.58   $21.60   $21.60  
  Purchase #5 $65.97   10.232%   $72.7201   $72.72   $72.70   $72.70  
  Purchase #6 $40.99   6.663%   $43.7212   $43.72   $43.70   $43.70  
  Purchase #7 $24.78   7.626%   $26.6697   $26.67   $26.65   $26.70  
  7 purchase totals = $292.91       $311.3045   $311.31   $311.25   $311.30