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 $27.91   3.137%   $28.7855   $28.79   $28.80   $28.80  
  Purchase #2 $21.28   7.756%   $22.9305   $22.93   $22.95   $22.90  
  Purchase #3 $130.99   13.162%   $148.2309   $148.23   $148.25   $148.20  
  Purchase #4 $201.29   9.663%   $220.7407   $220.74   $220.75   $220.70  
  Purchase #5 $152.26   4.429%   $159.0036   $159.00   $159.00   $159.00  
  Purchase #6 $208.68   7.581%   $224.5   $224.50   $224.50   $224.50  
  Purchase #7 $38.96   7.136%   $41.7402   $41.74   $41.75   $41.70  
  7 purchase totals = $781.37       $845.9314   $845.93   $846.00   $845.80