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 $78.39   6.453%   $83.4485   $83.45   $83.45   $83.40  
  Purchase #2 $171.69   9.500%   $188.0006   $188.00   $188.00   $188.00  
  Purchase #3 $166.97   12.728%   $188.2219   $188.22   $188.20   $188.20  
  Purchase #4 $141.38   7.334%   $151.7488   $151.75   $151.75   $151.70  
  Purchase #5 $65.71   7.538%   $70.6632   $70.66   $70.65   $70.70  
  Purchase #6 $146.72   4.655%   $153.5498   $153.55   $153.55   $153.50  
  Purchase #7 $46.84   13.409%   $53.1208   $53.12   $53.10   $53.10  
  7 purchase totals = $817.70       $888.7536   $888.75   $888.70   $888.60