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 $234.74   12.334%   $263.6928   $263.69   $263.70   $263.70  
  Purchase #2 $206.61   13.637%   $234.7854   $234.79   $234.80   $234.80  
  Purchase #3 $135.62   13.324%   $153.69   $153.69   $153.70   $153.70  
  Purchase #4 $185.29   13.212%   $209.7705   $209.77   $209.75   $209.80  
  Purchase #5 $124.45   10.274%   $137.236   $137.24   $137.25   $137.20  
  Purchase #6 $87.58   8.863%   $95.3422   $95.34   $95.35   $95.30  
  Purchase #7 $120.79   9.647%   $132.4426   $132.44   $132.45   $132.40  
  7 purchase totals = $1,095.08       $1,226.9595   $1,226.96   $1,227.00   $1,226.90