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 $17.57   7.465%   $18.8816   $18.88   $18.90   $18.90  
  Purchase #2 $202.52   6.867%   $216.427   $216.43   $216.45   $216.40  
  Purchase #3 $206.99   9.281%   $226.2007   $226.20   $226.20   $226.20  
  Purchase #4 $40.78   10.813%   $45.1895   $45.19   $45.20   $45.20  
  Purchase #5 $28.56   5.303%   $30.0745   $30.07   $30.05   $30.10  
  Purchase #6 $152.27   4.804%   $159.5851   $159.59   $159.60   $159.60  
  Purchase #7 $180.55   6.844%   $192.9068   $192.91   $192.90   $192.90  
  7 purchase totals = $829.24       $889.2652   $889.27   $889.30   $889.30