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 $239.53   13.121%   $270.9587   $270.96   $270.95   $271.00  
  Purchase #2 $213.93   13.385%   $242.5645   $242.56   $242.55   $242.60  
  Purchase #3 $234.81   6.148%   $249.2461   $249.25   $249.25   $249.20  
  Purchase #4 $53.53   11.294%   $59.5757   $59.58   $59.60   $59.60  
  Purchase #5 $158.52   6.831%   $169.3485   $169.35   $169.35   $169.30  
  Purchase #6 $98.82   9.138%   $107.8502   $107.85   $107.85   $107.90  
  Purchase #7 $111.85   9.452%   $122.4221   $122.42   $122.40   $122.40  
  7 purchase totals = $1,110.99       $1,221.9658   $1,221.97   $1,221.95   $1,222.00