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 $247.39   13.158%   $279.9416   $279.94   $279.95   $279.90  
  Purchase #2 $171.69   10.579%   $189.8531   $189.85   $189.85   $189.90  
  Purchase #3 $64.91   11.798%   $72.5681   $72.57   $72.55   $72.60  
  Purchase #4 $69.29   7.517%   $74.4985   $74.50   $74.50   $74.50  
  Purchase #5 $60.42   7.609%   $65.0174   $65.02   $65.00   $65.00  
  Purchase #6 $128.84   5.161%   $135.4894   $135.49   $135.50   $135.50  
  Purchase #7 $243.65   9.571%   $266.9697   $266.97   $266.95   $267.00  
  7 purchase totals = $986.19       $1,084.3378   $1,084.34   $1,084.30   $1,084.40