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 $61.97   3.606%   $64.2046   $64.20   $64.20   $64.20  
  Purchase #2 $229.33   8.549%   $248.9354   $248.94   $248.95   $248.90  
  Purchase #3 $134.54   8.850%   $146.4468   $146.45   $146.45   $146.40  
  Purchase #4 $98.81   11.372%   $110.0467   $110.05   $110.05   $110.00  
  Purchase #5 $85.60   7.798%   $92.2751   $92.28   $92.30   $92.30  
  Purchase #6 $41.59   11.842%   $46.5151   $46.52   $46.50   $46.50  
  Purchase #7 $181.56   10.580%   $200.769   $200.77   $200.75   $200.80  
  7 purchase totals = $833.40       $909.1927   $909.21   $909.20   $909.10