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 $240.56   11.819%   $268.9918   $268.99   $269.00   $269.00  
  Purchase #2 $155.79   5.519%   $164.3881   $164.39   $164.40   $164.40  
  Purchase #3 $249.51   13.745%   $283.8051   $283.81   $283.80   $283.80  
  Purchase #4 $239.67   3.363%   $247.7301   $247.73   $247.75   $247.70  
  Purchase #5 $97.87   5.602%   $103.3527   $103.35   $103.35   $103.40  
  Purchase #6 $48.26   3.553%   $49.9747   $49.97   $49.95   $50.00  
  Purchase #7 $133.53   3.395%   $138.0633   $138.06   $138.05   $138.10  
  7 purchase totals = $1,165.19       $1,256.3058   $1,256.30   $1,256.30   $1,256.40