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 $234.54   9.199%   $256.1153   $256.12   $256.10   $256.10  
  Purchase #2 $62.39   4.124%   $64.963   $64.96   $64.95   $65.00  
  Purchase #3 $190.65   7.424%   $204.8039   $204.80   $204.80   $204.80  
  Purchase #4 $27.93   7.862%   $30.1259   $30.13   $30.15   $30.10  
  Purchase #5 $10.53   9.630%   $11.544   $11.54   $11.55   $11.50  
  Purchase #6 $203.69   8.647%   $221.3031   $221.30   $221.30   $221.30  
  Purchase #7 $38.65   9.664%   $42.3851   $42.39   $42.40   $42.40  
  7 purchase totals = $768.38       $831.2403   $831.24   $831.25   $831.20