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 $13.46   12.318%   $15.118   $15.12   $15.10   $15.10  
  Purchase #2 $77.61   8.714%   $84.3729   $84.37   $84.35   $84.40  
  Purchase #3 $239.70   8.705%   $260.5659   $260.57   $260.55   $260.60  
  Purchase #4 $180.77   4.742%   $189.3421   $189.34   $189.35   $189.30  
  Purchase #5 $137.58   13.539%   $156.207   $156.21   $156.20   $156.20  
  Purchase #6 $89.67   10.608%   $99.1822   $99.18   $99.20   $99.20  
  Purchase #7 $118.98   7.677%   $128.1141   $128.11   $128.10   $128.10  
  7 purchase totals = $857.77       $932.9022   $932.90   $932.85   $932.90