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 $13.72   10.626%   $15.1779   $15.18   $15.20   $15.20  
  Purchase #2 $9.36   3.220%   $9.6614   $9.66   $9.65   $9.70  
  Purchase #3 $167.36   12.505%   $188.2884   $188.29   $188.30   $188.30  
  Purchase #4 $106.28   12.297%   $119.3493   $119.35   $119.35   $119.30  
  Purchase #5 $213.68   11.690%   $238.6592   $238.66   $238.65   $238.70  
  Purchase #6 $217.69   9.761%   $238.9387   $238.94   $238.95   $238.90  
  Purchase #7 $162.89   4.648%   $170.4611   $170.46   $170.45   $170.50  
  7 purchase totals = $890.98       $980.5360   $980.54   $980.55   $980.60