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 $128.65   9.433%   $140.7856   $140.79   $140.80   $140.80  
  Purchase #2 $123.30   3.151%   $127.1852   $127.19   $127.20   $127.20  
  Purchase #3 $77.87   7.615%   $83.7998   $83.80   $83.80   $83.80  
  Purchase #4 $29.83   12.505%   $33.5602   $33.56   $33.55   $33.60  
  Purchase #5 $76.50   9.403%   $83.6933   $83.69   $83.70   $83.70  
  Purchase #6 $96.29   6.455%   $102.5055   $102.51   $102.50   $102.50  
  Purchase #7 $7.93   3.418%   $8.201   $8.20   $8.20   $8.20  
  7 purchase totals = $540.37       $579.7306   $579.74   $579.75   $579.80