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 $227.33   12.447%   $255.6258   $255.63   $255.65   $255.60  
  Purchase #2 $31.70   10.269%   $34.9553   $34.96   $34.95   $35.00  
  Purchase #3 $62.86   4.824%   $65.8924   $65.89   $65.90   $65.90  
  Purchase #4 $95.78   6.288%   $101.8026   $101.80   $101.80   $101.80  
  Purchase #5 $185.42   11.231%   $206.2445   $206.24   $206.25   $206.20  
  Purchase #6 $246.82   12.702%   $278.1711   $278.17   $278.15   $278.20  
  Purchase #7 $59.99   3.336%   $61.9913   $61.99   $62.00   $62.00  
  7 purchase totals = $909.90       $1,004.6830   $1,004.68   $1,004.70   $1,004.70