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 $29.68   10.182%   $32.702   $32.70   $32.70   $32.70  
  Purchase #2 $198.46   13.292%   $224.8393   $224.84   $224.85   $224.80  
  Purchase #3 $176.93   12.736%   $199.4638   $199.46   $199.45   $199.50  
  Purchase #4 $129.68   11.618%   $144.7462   $144.75   $144.75   $144.70  
  Purchase #5 $29.66   5.545%   $31.3046   $31.30   $31.30   $31.30  
  Purchase #6 $101.70   5.257%   $107.0464   $107.05   $107.05   $107.00  
  Purchase #7 $173.83   13.691%   $197.6291   $197.63   $197.65   $197.60  
  7 purchase totals = $839.94       $937.7314   $937.73   $937.75   $937.60