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 $123.36   9.489%   $135.0656   $135.07   $135.05   $135.10  
  Purchase #2 $6.87   13.748%   $7.8145   $7.81   $7.80   $7.80  
  Purchase #3 $213.59   11.342%   $237.8154   $237.82   $237.80   $237.80  
  Purchase #4 $233.43   3.302%   $241.1379   $241.14   $241.15   $241.10  
  Purchase #5 $179.77   4.343%   $187.5774   $187.58   $187.60   $187.60  
  Purchase #6 $9.32   6.184%   $9.8963   $9.90   $9.90   $9.90  
  Purchase #7 $10.43   6.216%   $11.0783   $11.08   $11.10   $11.10  
  7 purchase totals = $776.77       $830.3854   $830.40   $830.40   $830.40