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 $204.61   12.244%   $229.6624   $229.66   $229.65   $229.70  
  Purchase #2 $100.53   9.726%   $110.3075   $110.31   $110.30   $110.30  
  Purchase #3 $245.47   8.860%   $267.2186   $267.22   $267.20   $267.20  
  Purchase #4 $164.62   13.203%   $186.3548   $186.35   $186.35   $186.40  
  Purchase #5 $127.95   9.707%   $140.3701   $140.37   $140.35   $140.40  
  Purchase #6 $104.33   12.344%   $117.2085   $117.21   $117.20   $117.20  
  Purchase #7 $196.85   11.392%   $219.2752   $219.28   $219.30   $219.30  
  7 purchase totals = $1,144.36       $1,270.3971   $1,270.40   $1,270.35   $1,270.50