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 $27.97   7.817%   $30.1564   $30.16   $30.15   $30.20  
  Purchase #2 $98.38   4.180%   $102.4923   $102.49   $102.50   $102.50  
  Purchase #3 $133.81   11.276%   $148.8984   $148.90   $148.90   $148.90  
  Purchase #4 $50.71   13.240%   $57.424   $57.42   $57.40   $57.40  
  Purchase #5 $97.41   10.531%   $107.6682   $107.67   $107.65   $107.70  
  Purchase #6 $28.85   9.622%   $31.6259   $31.63   $31.65   $31.60  
  Purchase #7 $185.75   4.453%   $194.0214   $194.02   $194.00   $194.00  
  7 purchase totals = $622.88       $672.2866   $672.29   $672.25   $672.30