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 $15.80   12.770%   $17.8177   $17.82   $17.80   $17.80  
  Purchase #2 $15.96   7.342%   $17.1318   $17.13   $17.15   $17.10  
  Purchase #3 $60.84   9.580%   $66.6685   $66.67   $66.65   $66.70  
  Purchase #4 $167.26   4.181%   $174.2531   $174.25   $174.25   $174.30  
  Purchase #5 $35.97   12.375%   $40.4213   $40.42   $40.40   $40.40  
  Purchase #6 $13.43   11.527%   $14.9781   $14.98   $15.00   $15.00  
  Purchase #7 $82.70   4.855%   $86.7151   $86.72   $86.70   $86.70  
  7 purchase totals = $391.96       $417.9856   $417.99   $417.95   $418.00