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 $28.60   3.507%   $29.603   $29.60   $29.60   $29.60  
  Purchase #2 $37.97   5.323%   $39.9911   $39.99   $40.00   $40.00  
  Purchase #3 $203.55   11.481%   $226.9196   $226.92   $226.90   $226.90  
  Purchase #4 $28.89   7.392%   $31.0255   $31.03   $31.05   $31.00  
  Purchase #5 $189.94   8.671%   $206.4097   $206.41   $206.40   $206.40  
  Purchase #6 $41.99   7.133%   $44.9851   $44.99   $45.00   $45.00  
  Purchase #7 $143.82   3.645%   $149.0622   $149.06   $149.05   $149.10  
  7 purchase totals = $674.76       $727.9962   $728.00   $728.00   $728.00