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 $246.93   12.163%   $276.9641   $276.96   $276.95   $277.00  
  Purchase #2 $165.25   6.724%   $176.3614   $176.36   $176.35   $176.40  
  Purchase #3 $48.75   8.444%   $52.8665   $52.87   $52.85   $52.90  
  Purchase #4 $142.29   3.341%   $147.0439   $147.04   $147.05   $147.00  
  Purchase #5 $235.76   8.709%   $256.2923   $256.29   $256.30   $256.30  
  Purchase #6 $203.91   8.606%   $221.4585   $221.46   $221.45   $221.50  
  Purchase #7 $107.49   9.795%   $118.0186   $118.02   $118.00   $118.00  
  7 purchase totals = $1,150.38       $1,249.0053   $1,249.00   $1,248.95   $1,249.10