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 $233.63   10.287%   $257.6635   $257.66   $257.65   $257.70  
  Purchase #2 $26.62   12.672%   $29.9933   $29.99   $30.00   $30.00  
  Purchase #3 $155.93   8.332%   $168.9221   $168.92   $168.90   $168.90  
  Purchase #4 $36.47   8.583%   $39.6002   $39.60   $39.60   $39.60  
  Purchase #5 $87.79   13.148%   $99.3326   $99.33   $99.35   $99.30  
  Purchase #6 $5.86   12.707%   $6.6046   $6.60   $6.60   $6.60  
  Purchase #7 $189.56   6.856%   $202.5562   $202.56   $202.55   $202.60  
  7 purchase totals = $735.86       $804.6725   $804.66   $804.65   $804.70