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 $246.32   9.826%   $270.5234   $270.52   $270.50   $270.50  
  Purchase #2 $216.46   4.565%   $226.3414   $226.34   $226.35   $226.30  
  Purchase #3 $167.57   3.586%   $173.5791   $173.58   $173.60   $173.60  
  Purchase #4 $182.98   3.419%   $189.2361   $189.24   $189.25   $189.20  
  Purchase #5 $113.59   9.317%   $124.1732   $124.17   $124.15   $124.20  
  Purchase #6 $222.73   12.318%   $250.1659   $250.17   $250.15   $250.20  
  Purchase #7 $60.79   5.134%   $63.911   $63.91   $63.90   $63.90  
  7 purchase totals = $1,210.44       $1,297.9301   $1,297.93   $1,297.90   $1,297.90