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 $146.66   12.725%   $165.3225   $165.32   $165.30   $165.30  
  Purchase #2 $26.93   9.603%   $29.5161   $29.52   $29.50   $29.50  
  Purchase #3 $114.99   11.292%   $127.9747   $127.97   $127.95   $128.00  
  Purchase #4 $31.94   5.358%   $33.6513   $33.65   $33.65   $33.70  
  Purchase #5 $209.59   13.703%   $238.3101   $238.31   $238.30   $238.30  
  Purchase #6 $175.74   12.186%   $197.1557   $197.16   $197.15   $197.20  
  Purchase #7 $60.68   5.500%   $64.0174   $64.02   $64.00   $64.00  
  7 purchase totals = $766.53       $855.9478   $855.95   $855.85   $856.00