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 $28.46   5.869%   $30.1303   $30.13   $30.15   $30.10  
  Purchase #2 $90.64   10.374%   $100.043   $100.04   $100.05   $100.00  
  Purchase #3 $113.80   7.165%   $121.9538   $121.95   $121.95   $122.00  
  Purchase #4 $11.29   6.154%   $11.9848   $11.98   $12.00   $12.00  
  Purchase #5 $50.36   13.447%   $57.1319   $57.13   $57.15   $57.10  
  Purchase #6 $53.26   5.666%   $56.2777   $56.28   $56.30   $56.30  
  Purchase #7 $234.41   10.174%   $258.2589   $258.26   $258.25   $258.30  
  7 purchase totals = $582.22       $635.7804   $635.77   $635.85   $635.80