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 $241.69   12.176%   $271.1182   $271.12   $271.10   $271.10  
  Purchase #2 $23.54   7.702%   $25.3531   $25.35   $25.35   $25.40  
  Purchase #3 $88.63   8.515%   $96.1768   $96.18   $96.20   $96.20  
  Purchase #4 $224.27   4.297%   $233.9069   $233.91   $233.90   $233.90  
  Purchase #5 $25.76   9.766%   $28.2757   $28.28   $28.30   $28.30  
  Purchase #6 $209.60   10.691%   $232.0083   $232.01   $232.00   $232.00  
  Purchase #7 $124.94   8.275%   $135.2788   $135.28   $135.30   $135.30  
  7 purchase totals = $938.43       $1,022.1178   $1,022.13   $1,022.15   $1,022.20