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 $61.68   3.518%   $63.8499   $63.85   $63.85   $63.80  
  Purchase #2 $5.42   7.218%   $5.8112   $5.81   $5.80   $5.80  
  Purchase #3 $45.77   12.211%   $51.359   $51.36   $51.35   $51.40  
  Purchase #4 $124.40   6.463%   $132.44   $132.44   $132.45   $132.40  
  Purchase #5 $127.25   8.463%   $138.0192   $138.02   $138.00   $138.00  
  Purchase #6 $196.65   3.868%   $204.2564   $204.26   $204.25   $204.30  
  Purchase #7 $220.35   3.200%   $227.4012   $227.40   $227.40   $227.40  
  7 purchase totals = $781.52       $823.1369   $823.14   $823.10   $823.10