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 $93.73   11.771%   $104.763   $104.76   $104.75   $104.80  
  Purchase #2 $142.26   6.135%   $150.9877   $150.99   $151.00   $151.00  
  Purchase #3 $26.50   5.504%   $27.9586   $27.96   $27.95   $28.00  
  Purchase #4 $220.42   12.653%   $248.3097   $248.31   $248.30   $248.30  
  Purchase #5 $203.61   7.321%   $218.5163   $218.52   $218.50   $218.50  
  Purchase #6 $160.57   4.777%   $168.2404   $168.24   $168.25   $168.20  
  Purchase #7 $53.67   12.670%   $60.47   $60.47   $60.45   $60.50  
  7 purchase totals = $900.76       $979.2457   $979.25   $979.20   $979.30