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 $28.90   5.191%   $30.4002   $30.40   $30.40   $30.40  
  Purchase #2 $107.80   8.228%   $116.6698   $116.67   $116.65   $116.70  
  Purchase #3 $99.97   7.822%   $107.7897   $107.79   $107.80   $107.80  
  Purchase #4 $7.95   9.245%   $8.685   $8.69   $8.70   $8.70  
  Purchase #5 $8.85   10.541%   $9.7829   $9.78   $9.80   $9.80  
  Purchase #6 $172.45   4.312%   $179.886   $179.89   $179.90   $179.90  
  Purchase #7 $94.87   9.544%   $103.9244   $103.92   $103.90   $103.90  
  7 purchase totals = $520.79       $557.1380   $557.14   $557.15   $557.20