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 $15.95   12.707%   $17.9768   $17.98   $18.00   $18.00  
  Purchase #2 $175.74   3.442%   $181.789   $181.79   $181.80   $181.80  
  Purchase #3 $12.96   13.290%   $14.6824   $14.68   $14.70   $14.70  
  Purchase #4 $114.84   5.352%   $120.9862   $120.99   $121.00   $121.00  
  Purchase #5 $62.59   5.704%   $66.1601   $66.16   $66.15   $66.20  
  Purchase #6 $154.89   5.173%   $162.9025   $162.90   $162.90   $162.90  
  Purchase #7 $40.83   10.771%   $45.2278   $45.23   $45.25   $45.20  
  7 purchase totals = $577.80       $609.7248   $609.73   $609.80   $609.80