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 $220.85   13.624%   $250.9386   $250.94   $250.95   $250.90  
  Purchase #2 $149.42   11.702%   $166.9051   $166.91   $166.90   $166.90  
  Purchase #3 $35.97   3.159%   $37.1063   $37.11   $37.10   $37.10  
  Purchase #4 $170.63   5.805%   $180.5351   $180.54   $180.55   $180.50  
  Purchase #5 $62.30   4.184%   $64.9066   $64.91   $64.90   $64.90  
  Purchase #6 $10.81   12.397%   $12.1501   $12.15   $12.15   $12.20  
  Purchase #7 $231.53   8.652%   $251.562   $251.56   $251.55   $251.60  
  7 purchase totals = $881.51       $964.1038   $964.12   $964.10   $964.10