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 $7.74   7.136%   $8.2923   $8.29   $8.30   $8.30  
  Purchase #2 $104.80   3.533%   $108.5026   $108.50   $108.50   $108.50  
  Purchase #3 $7.53   11.270%   $8.3786   $8.38   $8.40   $8.40  
  Purchase #4 $76.36   6.735%   $81.5028   $81.50   $81.50   $81.50  
  Purchase #5 $72.83   9.494%   $79.7445   $79.74   $79.75   $79.70  
  Purchase #6 $43.53   3.348%   $44.9874   $44.99   $45.00   $45.00  
  Purchase #7 $184.39   8.602%   $200.2512   $200.25   $200.25   $200.30  
  7 purchase totals = $497.18       $531.6594   $531.65   $531.70   $531.70