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 $60.92   12.115%   $68.3005   $68.30   $68.30   $68.30  
  Purchase #2 $169.53   8.330%   $183.6518   $183.65   $183.65   $183.70  
  Purchase #3 $227.78   5.447%   $240.1872   $240.19   $240.20   $240.20  
  Purchase #4 $32.43   3.493%   $33.5628   $33.56   $33.55   $33.60  
  Purchase #5 $224.45   3.651%   $232.6447   $232.64   $232.65   $232.60  
  Purchase #6 $179.61   9.701%   $197.034   $197.03   $197.05   $197.00  
  Purchase #7 $21.58   7.291%   $23.1534   $23.15   $23.15   $23.20  
  7 purchase totals = $916.30       $978.5344   $978.52   $978.55   $978.60