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 $61.34   8.182%   $66.3588   $66.36   $66.35   $66.40  
  Purchase #2 $204.74   11.171%   $227.6115   $227.61   $227.60   $227.60  
  Purchase #3 $247.61   7.188%   $265.4082   $265.41   $265.40   $265.40  
  Purchase #4 $37.93   10.310%   $41.8406   $41.84   $41.85   $41.80  
  Purchase #5 $120.85   10.134%   $133.0969   $133.10   $133.10   $133.10  
  Purchase #6 $73.57   4.417%   $76.8196   $76.82   $76.80   $76.80  
  Purchase #7 $88.32   8.440%   $95.7742   $95.77   $95.75   $95.80  
  7 purchase totals = $834.36       $906.9098   $906.91   $906.85   $906.90