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 $5.69   6.141%   $6.0394   $6.04   $6.05   $6.00  
  Purchase #2 $186.95   4.450%   $195.2693   $195.27   $195.25   $195.30  
  Purchase #3 $142.56   7.323%   $152.9997   $153.00   $153.00   $153.00  
  Purchase #4 $101.39   9.418%   $110.9389   $110.94   $110.95   $110.90  
  Purchase #5 $51.95   3.280%   $53.654   $53.65   $53.65   $53.70  
  Purchase #6 $46.82   9.600%   $51.3147   $51.31   $51.30   $51.30  
  Purchase #7 $123.91   13.698%   $140.8832   $140.88   $140.90   $140.90  
  7 purchase totals = $659.27       $711.0992   $711.09   $711.10   $711.10