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 $19.86   5.645%   $20.9811   $20.98   $21.00   $21.00  
  Purchase #2 $53.48   8.871%   $58.2242   $58.22   $58.20   $58.20  
  Purchase #3 $154.77   4.810%   $162.2144   $162.21   $162.20   $162.20  
  Purchase #4 $171.40   12.539%   $192.8918   $192.89   $192.90   $192.90  
  Purchase #5 $234.75   12.703%   $264.5703   $264.57   $264.55   $264.60  
  Purchase #6 $64.90   7.870%   $70.0076   $70.01   $70.00   $70.00  
  Purchase #7 $100.44   13.414%   $113.913   $113.91   $113.90   $113.90  
  7 purchase totals = $799.60       $882.8024   $882.79   $882.75   $882.80