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 $28.68   8.685%   $31.1709   $31.17   $31.15   $31.20  
  Purchase #2 $182.91   3.752%   $189.7728   $189.77   $189.75   $189.80  
  Purchase #3 $142.73   13.261%   $161.6574   $161.66   $161.65   $161.70  
  Purchase #4 $49.53   6.578%   $52.7881   $52.79   $52.80   $52.80  
  Purchase #5 $28.54   4.303%   $29.7681   $29.77   $29.75   $29.80  
  Purchase #6 $32.67   4.508%   $34.1428   $34.14   $34.15   $34.10  
  Purchase #7 $20.77   3.704%   $21.5393   $21.54   $21.55   $21.50  
  7 purchase totals = $485.83       $520.8394   $520.84   $520.80   $520.90