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 $27.80   13.304%   $31.4985   $31.50   $31.50   $31.50  
  Purchase #2 $190.97   5.453%   $201.3836   $201.38   $201.40   $201.40  
  Purchase #3 $28.92   8.683%   $31.4311   $31.43   $31.45   $31.40  
  Purchase #4 $218.57   13.488%   $248.0507   $248.05   $248.05   $248.10  
  Purchase #5 $70.45   12.540%   $79.2844   $79.28   $79.30   $79.30  
  Purchase #6 $49.72   10.605%   $54.9928   $54.99   $55.00   $55.00  
  Purchase #7 $100.42   3.757%   $104.1928   $104.19   $104.20   $104.20  
  7 purchase totals = $686.85       $750.8339   $750.82   $750.90   $750.90