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 $136.87   11.777%   $152.9892   $152.99   $153.00   $153.00  
  Purchase #2 $219.74   9.216%   $239.9912   $239.99   $240.00   $240.00  
  Purchase #3 $89.52   6.436%   $95.2815   $95.28   $95.30   $95.30  
  Purchase #4 $241.99   6.595%   $257.9492   $257.95   $257.95   $257.90  
  Purchase #5 $179.95   11.457%   $200.5669   $200.57   $200.55   $200.60  
  Purchase #6 $32.30   8.221%   $34.9554   $34.96   $34.95   $35.00  
  Purchase #7 $43.41   12.126%   $48.6739   $48.67   $48.65   $48.70  
  7 purchase totals = $943.78       $1,030.4073   $1,030.41   $1,030.40   $1,030.50