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 $16.57   13.476%   $18.803   $18.80   $18.80   $18.80  
  Purchase #2 $222.62   9.502%   $243.7734   $243.77   $243.75   $243.80  
  Purchase #3 $199.54   9.645%   $218.7856   $218.79   $218.80   $218.80  
  Purchase #4 $10.99   10.418%   $12.1349   $12.13   $12.15   $12.10  
  Purchase #5 $175.36   9.206%   $191.5036   $191.50   $191.50   $191.50  
  Purchase #6 $155.88   5.700%   $164.7652   $164.77   $164.75   $164.80  
  Purchase #7 $189.34   8.190%   $204.8469   $204.85   $204.85   $204.80  
  7 purchase totals = $970.30       $1,054.6126   $1,054.61   $1,054.60   $1,054.60