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 $64.53   3.644%   $66.8815   $66.88   $66.90   $66.90  
  Purchase #2 $149.99   11.542%   $167.3018   $167.30   $167.30   $167.30  
  Purchase #3 $32.64   6.361%   $34.7162   $34.72   $34.70   $34.70  
  Purchase #4 $107.29   9.771%   $117.7733   $117.77   $117.75   $117.80  
  Purchase #5 $235.72   12.504%   $265.1944   $265.19   $265.20   $265.20  
  Purchase #6 $133.69   13.453%   $151.6753   $151.68   $151.70   $151.70  
  Purchase #7 $38.30   8.583%   $41.5873   $41.59   $41.60   $41.60  
  7 purchase totals = $762.16       $845.1298   $845.13   $845.15   $845.20