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 $138.65   10.740%   $153.541   $153.54   $153.55   $153.50  
  Purchase #2 $249.26   8.130%   $269.5248   $269.52   $269.50   $269.50  
  Purchase #3 $124.72   10.586%   $137.9229   $137.92   $137.90   $137.90  
  Purchase #4 $163.77   10.391%   $180.7873   $180.79   $180.80   $180.80  
  Purchase #5 $95.38   13.625%   $108.3755   $108.38   $108.40   $108.40  
  Purchase #6 $212.59   11.403%   $236.8316   $236.83   $236.85   $236.80  
  Purchase #7 $241.87   11.635%   $270.0116   $270.01   $270.00   $270.00  
  7 purchase totals = $1,226.24       $1,356.9947   $1,356.99   $1,357.00   $1,356.90