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 $134.57   4.426%   $140.5261   $140.53   $140.55   $140.50  
  Purchase #2 $75.82   7.432%   $81.4549   $81.45   $81.45   $81.50  
  Purchase #3 $93.25   5.691%   $98.5569   $98.56   $98.55   $98.60  
  Purchase #4 $83.25   8.496%   $90.3229   $90.32   $90.30   $90.30  
  Purchase #5 $98.93   11.379%   $110.1872   $110.19   $110.20   $110.20  
  Purchase #6 $151.88   7.672%   $163.5322   $163.53   $163.55   $163.50  
  Purchase #7 $166.67   13.201%   $188.6721   $188.67   $188.65   $188.70  
  7 purchase totals = $804.37       $873.2523   $873.25   $873.25   $873.30