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 $28.63   8.696%   $31.1197   $31.12   $31.10   $31.10  
  Purchase #2 $31.84   8.772%   $34.633   $34.63   $34.65   $34.60  
  Purchase #3 $104.63   13.256%   $118.4998   $118.50   $118.50   $118.50  
  Purchase #4 $123.97   7.748%   $133.5752   $133.58   $133.60   $133.60  
  Purchase #5 $139.38   4.620%   $145.8194   $145.82   $145.80   $145.80  
  Purchase #6 $121.32   5.729%   $128.2704   $128.27   $128.25   $128.30  
  Purchase #7 $44.97   12.854%   $50.7504   $50.75   $50.75   $50.80  
  7 purchase totals = $594.74       $642.6679   $642.67   $642.65   $642.70