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 $238.80   8.622%   $259.3893   $259.39   $259.40   $259.40  
  Purchase #2 $30.78   10.513%   $34.0159   $34.02   $34.00   $34.00  
  Purchase #3 $120.55   3.597%   $124.8862   $124.89   $124.90   $124.90  
  Purchase #4 $81.81   7.503%   $87.9482   $87.95   $87.95   $87.90  
  Purchase #5 $218.85   10.159%   $241.083   $241.08   $241.10   $241.10  
  Purchase #6 $133.80   10.552%   $147.9186   $147.92   $147.90   $147.90  
  Purchase #7 $141.27   6.178%   $149.9977   $150.00   $150.00   $150.00  
  7 purchase totals = $965.86       $1,045.2389   $1,045.25   $1,045.25   $1,045.20