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 $240.28   7.816%   $259.0603   $259.06   $259.05   $259.10  
  Purchase #2 $202.87   12.867%   $228.9733   $228.97   $228.95   $229.00  
  Purchase #3 $243.44   10.759%   $269.6317   $269.63   $269.65   $269.60  
  Purchase #4 $106.47   5.773%   $112.6165   $112.62   $112.60   $112.60  
  Purchase #5 $8.50   11.481%   $9.4759   $9.48   $9.50   $9.50  
  Purchase #6 $67.61   4.315%   $70.5274   $70.53   $70.55   $70.50  
  Purchase #7 $124.89   12.810%   $140.8884   $140.89   $140.90   $140.90  
  7 purchase totals = $994.06       $1,091.1735   $1,091.18   $1,091.20   $1,091.20