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 $6.99   12.570%   $7.8686   $7.87   $7.85   $7.90  
  Purchase #2 $177.72   9.562%   $194.7136   $194.71   $194.70   $194.70  
  Purchase #3 $151.77   5.351%   $159.8912   $159.89   $159.90   $159.90  
  Purchase #4 $73.69   10.396%   $81.3508   $81.35   $81.35   $81.40  
  Purchase #5 $31.55   12.262%   $35.4187   $35.42   $35.40   $35.40  
  Purchase #6 $111.56   9.237%   $121.8648   $121.86   $121.85   $121.90  
  Purchase #7 $44.74   3.181%   $46.1632   $46.16   $46.15   $46.20  
  7 purchase totals = $598.02       $647.2709   $647.26   $647.20   $647.40