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 $19.76   7.137%   $21.1703   $21.17   $21.15   $21.20  
  Purchase #2 $151.94   12.570%   $171.0389   $171.04   $171.05   $171.00  
  Purchase #3 $220.58   13.628%   $250.6406   $250.64   $250.65   $250.60  
  Purchase #4 $9.85   13.673%   $11.1968   $11.20   $11.20   $11.20  
  Purchase #5 $142.87   11.529%   $159.3415   $159.34   $159.35   $159.30  
  Purchase #6 $124.39   7.218%   $133.3685   $133.37   $133.35   $133.40  
  Purchase #7 $23.60   13.844%   $26.8672   $26.87   $26.85   $26.90  
  7 purchase totals = $692.99       $773.6238   $773.63   $773.60   $773.60