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 $234.95   13.324%   $266.2547   $266.25   $266.25   $266.30  
  Purchase #2 $62.72   5.644%   $66.2599   $66.26   $66.25   $66.30  
  Purchase #3 $95.96   5.189%   $100.9394   $100.94   $100.95   $100.90  
  Purchase #4 $202.82   7.763%   $218.5649   $218.56   $218.55   $218.60  
  Purchase #5 $168.42   9.241%   $183.9837   $183.98   $184.00   $184.00  
  Purchase #6 $120.37   12.156%   $135.0022   $135.00   $135.00   $135.00  
  Purchase #7 $52.94   8.156%   $57.2578   $57.26   $57.25   $57.30  
  7 purchase totals = $938.18       $1,028.2626   $1,028.25   $1,028.25   $1,028.40