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 $17.33   4.299%   $18.075   $18.08   $18.10   $18.10  
  Purchase #2 $166.25   9.663%   $182.3147   $182.31   $182.30   $182.30  
  Purchase #3 $29.96   4.482%   $31.3028   $31.30   $31.30   $31.30  
  Purchase #4 $102.30   12.259%   $114.841   $114.84   $114.85   $114.80  
  Purchase #5 $106.32   12.561%   $119.6749   $119.67   $119.65   $119.70  
  Purchase #6 $107.35   11.356%   $119.5407   $119.54   $119.55   $119.50  
  Purchase #7 $94.97   3.383%   $98.1828   $98.18   $98.20   $98.20  
  7 purchase totals = $624.48       $683.9319   $683.92   $683.95   $683.90