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 $124.51   11.237%   $138.5012   $138.50   $138.50   $138.50  
  Purchase #2 $98.38   6.393%   $104.6694   $104.67   $104.65   $104.70  
  Purchase #3 $245.86   5.449%   $259.2569   $259.26   $259.25   $259.30  
  Purchase #4 $131.63   8.574%   $142.916   $142.92   $142.90   $142.90  
  Purchase #5 $209.96   7.275%   $225.2346   $225.23   $225.25   $225.20  
  Purchase #6 $223.66   12.788%   $252.2616   $252.26   $252.25   $252.30  
  Purchase #7 $225.79   6.790%   $241.1211   $241.12   $241.10   $241.10  
  7 purchase totals = $1,259.79       $1,363.9608   $1,363.96   $1,363.90   $1,364.00