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 $241.74   11.278%   $269.0034   $269.00   $269.00   $269.00  
  Purchase #2 $229.93   9.715%   $252.2677   $252.27   $252.25   $252.30  
  Purchase #3 $215.26   7.343%   $231.0665   $231.07   $231.05   $231.10  
  Purchase #4 $155.69   6.142%   $165.2525   $165.25   $165.25   $165.30  
  Purchase #5 $23.33   3.157%   $24.0665   $24.07   $24.05   $24.10  
  Purchase #6 $218.83   3.536%   $226.5678   $226.57   $226.55   $226.60  
  Purchase #7 $85.52   3.103%   $88.1737   $88.17   $88.15   $88.20  
  7 purchase totals = $1,170.30       $1,256.3981   $1,256.40   $1,256.30   $1,256.60