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 $8.28   13.386%   $9.3884   $9.39   $9.40   $9.40  
  Purchase #2 $9.53   6.678%   $10.1664   $10.17   $10.15   $10.20  
  Purchase #3 $169.64   4.401%   $177.1059   $177.11   $177.10   $177.10  
  Purchase #4 $127.78   4.384%   $133.3819   $133.38   $133.40   $133.40  
  Purchase #5 $178.27   5.793%   $188.5972   $188.60   $188.60   $188.60  
  Purchase #6 $7.73   9.286%   $8.4478   $8.45   $8.45   $8.40  
  Purchase #7 $195.96   3.250%   $202.3287   $202.33   $202.35   $202.30  
  7 purchase totals = $697.19       $729.4163   $729.43   $729.45   $729.40