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 $30.33   5.131%   $31.8862   $31.89   $31.90   $31.90  
  Purchase #2 $7.95   12.717%   $8.961   $8.96   $8.95   $9.00  
  Purchase #3 $138.73   5.165%   $145.8954   $145.90   $145.90   $145.90  
  Purchase #4 $46.53   12.799%   $52.4854   $52.49   $52.50   $52.50  
  Purchase #5 $25.37   7.156%   $27.1855   $27.19   $27.20   $27.20  
  Purchase #6 $109.94   8.293%   $119.0573   $119.06   $119.05   $119.10  
  Purchase #7 $209.42   3.273%   $216.2743   $216.27   $216.25   $216.30  
  7 purchase totals = $568.27       $601.7451   $601.76   $601.75   $601.90