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 $246.94   12.401%   $277.563   $277.56   $277.55   $277.60  
  Purchase #2 $80.35   6.116%   $85.2642   $85.26   $85.25   $85.30  
  Purchase #3 $227.73   7.141%   $243.9922   $243.99   $244.00   $244.00  
  Purchase #4 $67.29   5.704%   $71.1282   $71.13   $71.15   $71.10  
  Purchase #5 $7.29   9.722%   $7.9987   $8.00   $8.00   $8.00  
  Purchase #6 $220.62   6.403%   $234.7463   $234.75   $234.75   $234.70  
  Purchase #7 $84.53   9.490%   $92.5519   $92.55   $92.55   $92.60  
  7 purchase totals = $934.75       $1,013.2445   $1,013.24   $1,013.25   $1,013.30