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 $65.78   4.531%   $68.7605   $68.76   $68.75   $68.80  
  Purchase #2 $49.33   9.169%   $53.8531   $53.85   $53.85   $53.90  
  Purchase #3 $114.88   6.135%   $121.9279   $121.93   $121.95   $121.90  
  Purchase #4 $243.38   11.181%   $270.5923   $270.59   $270.60   $270.60  
  Purchase #5 $140.39   11.573%   $156.6373   $156.64   $156.65   $156.60  
  Purchase #6 $47.72   4.742%   $49.9829   $49.98   $50.00   $50.00  
  Purchase #7 $104.38   11.402%   $116.2814   $116.28   $116.30   $116.30  
  7 purchase totals = $765.86       $838.0354   $838.03   $838.10   $838.10