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 $11.89   9.207%   $12.9847   $12.98   $13.00   $13.00  
  Purchase #2 $227.77   5.470%   $240.229   $240.23   $240.25   $240.20  
  Purchase #3 $169.88   9.625%   $186.231   $186.23   $186.25   $186.20  
  Purchase #4 $158.95   12.818%   $179.3242   $179.32   $179.30   $179.30  
  Purchase #5 $188.96   8.115%   $204.2941   $204.29   $204.30   $204.30  
  Purchase #6 $75.52   5.497%   $79.6713   $79.67   $79.65   $79.70  
  Purchase #7 $166.58   11.495%   $185.7284   $185.73   $185.75   $185.70  
  7 purchase totals = $999.55       $1,088.4627   $1,088.45   $1,088.50   $1,088.40