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 $29.80   9.209%   $32.5443   $32.54   $32.55   $32.50  
  Purchase #2 $63.43   8.381%   $68.7461   $68.75   $68.75   $68.70  
  Purchase #3 $71.94   4.742%   $75.3514   $75.35   $75.35   $75.40  
  Purchase #4 $230.27   5.478%   $242.8842   $242.88   $242.90   $242.90  
  Purchase #5 $60.95   3.290%   $62.9553   $62.96   $62.95   $63.00  
  Purchase #6 $53.40   11.652%   $59.6222   $59.62   $59.60   $59.60  
  Purchase #7 $134.78   4.631%   $141.0217   $141.02   $141.00   $141.00  
  7 purchase totals = $644.57       $683.1252   $683.12   $683.10   $683.10