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 $118.81   12.753%   $133.9618   $133.96   $133.95   $134.00  
  Purchase #2 $75.52   10.839%   $83.7056   $83.71   $83.70   $83.70  
  Purchase #3 $65.35   11.706%   $72.9999   $73.00   $73.00   $73.00  
  Purchase #4 $61.85   11.621%   $69.0376   $69.04   $69.05   $69.00  
  Purchase #5 $98.29   4.621%   $102.832   $102.83   $102.85   $102.80  
  Purchase #6 $189.54   9.354%   $207.2696   $207.27   $207.25   $207.30  
  Purchase #7 $158.88   7.644%   $171.0248   $171.02   $171.00   $171.00  
  7 purchase totals = $768.24       $840.8313   $840.83   $840.80   $840.80