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 $28.80   4.388%   $30.0637   $30.06   $30.05   $30.10  
  Purchase #2 $13.46   6.700%   $14.3618   $14.36   $14.35   $14.40  
  Purchase #3 $77.32   11.295%   $86.0533   $86.05   $86.05   $86.10  
  Purchase #4 $224.64   7.708%   $241.9553   $241.96   $241.95   $242.00  
  Purchase #5 $54.60   7.339%   $58.6071   $58.61   $58.60   $58.60  
  Purchase #6 $146.42   9.730%   $160.6667   $160.67   $160.65   $160.70  
  Purchase #7 $93.46   12.470%   $105.1145   $105.11   $105.10   $105.10  
  7 purchase totals = $638.70       $696.8224   $696.82   $696.75   $697.00