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 $17.84   12.499%   $20.0698   $20.07   $20.05   $20.10  
  Purchase #2 $39.40   11.540%   $43.9468   $43.95   $43.95   $43.90  
  Purchase #3 $102.40   5.500%   $108.032   $108.03   $108.05   $108.00  
  Purchase #4 $142.32   3.574%   $147.4065   $147.41   $147.40   $147.40  
  Purchase #5 $125.91   3.753%   $130.6354   $130.64   $130.65   $130.60  
  Purchase #6 $33.59   6.157%   $35.6581   $35.66   $35.65   $35.70  
  Purchase #7 $157.90   10.560%   $174.5742   $174.57   $174.55   $174.60  
  7 purchase totals = $619.36       $660.3228   $660.33   $660.30   $660.30