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 $182.96   4.726%   $191.6067   $191.61   $191.60   $191.60  
  Purchase #2 $111.75   10.451%   $123.429   $123.43   $123.45   $123.40  
  Purchase #3 $44.86   11.311%   $49.9341   $49.93   $49.95   $49.90  
  Purchase #4 $200.66   12.653%   $226.0495   $226.05   $226.05   $226.00  
  Purchase #5 $92.31   4.798%   $96.739   $96.74   $96.75   $96.70  
  Purchase #6 $21.76   11.217%   $24.2008   $24.20   $24.20   $24.20  
  Purchase #7 $106.93   9.459%   $117.0445   $117.04   $117.05   $117.00  
  7 purchase totals = $761.23       $829.0036   $829.00   $829.05   $828.80