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 $135.47   11.715%   $151.3403   $151.34   $151.35   $151.30  
  Purchase #2 $185.93   11.147%   $206.6556   $206.66   $206.65   $206.70  
  Purchase #3 $34.95   13.674%   $39.7291   $39.73   $39.75   $39.70  
  Purchase #4 $88.25   13.378%   $100.0561   $100.06   $100.05   $100.10  
  Purchase #5 $220.42   7.371%   $236.6672   $236.67   $236.65   $236.70  
  Purchase #6 $209.96   9.719%   $230.366   $230.37   $230.35   $230.40  
  Purchase #7 $248.29   13.392%   $281.541   $281.54   $281.55   $281.50  
  7 purchase totals = $1,123.27       $1,246.3553   $1,246.37   $1,246.35   $1,246.40