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 $7.80   7.243%   $8.365   $8.37   $8.35   $8.40  
  Purchase #2 $190.82   12.550%   $214.7679   $214.77   $214.75   $214.80  
  Purchase #3 $233.91   10.303%   $258.0097   $258.01   $258.00   $258.00  
  Purchase #4 $153.78   4.566%   $160.8016   $160.80   $160.80   $160.80  
  Purchase #5 $103.88   12.756%   $117.1309   $117.13   $117.15   $117.10  
  Purchase #6 $31.83   9.377%   $34.8147   $34.81   $34.80   $34.80  
  Purchase #7 $124.85   8.239%   $135.1364   $135.14   $135.15   $135.10  
  7 purchase totals = $846.87       $929.0262   $929.03   $929.00   $929.00