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 $249.61   5.675%   $263.7754   $263.78   $263.80   $263.80  
  Purchase #2 $232.82   13.451%   $264.1366   $264.14   $264.15   $264.10  
  Purchase #3 $248.37   10.793%   $275.1766   $275.18   $275.20   $275.20  
  Purchase #4 $191.52   12.691%   $215.8258   $215.83   $215.85   $215.80  
  Purchase #5 $80.40   4.727%   $84.2005   $84.20   $84.20   $84.20  
  Purchase #6 $161.63   6.627%   $172.3412   $172.34   $172.35   $172.30  
  Purchase #7 $247.78   11.759%   $276.9165   $276.92   $276.90   $276.90  
  7 purchase totals = $1,412.13       $1,552.3726   $1,552.39   $1,552.45   $1,552.30