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 $249.75   10.638%   $276.3184   $276.32   $276.30   $276.30  
  Purchase #2 $188.94   6.185%   $200.6259   $200.63   $200.65   $200.60  
  Purchase #3 $182.35   9.683%   $200.007   $200.01   $200.00   $200.00  
  Purchase #4 $205.45   12.125%   $230.3608   $230.36   $230.35   $230.40  
  Purchase #5 $201.39   12.185%   $225.9294   $225.93   $225.95   $225.90  
  Purchase #6 $206.27   6.649%   $219.9849   $219.98   $220.00   $220.00  
  Purchase #7 $126.37   10.547%   $139.6982   $139.70   $139.70   $139.70  
  7 purchase totals = $1,360.52       $1,492.9246   $1,492.93   $1,492.95   $1,492.90