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 $20.78   3.322%   $21.4703   $21.47   $21.45   $21.50  
  Purchase #2 $170.33   6.150%   $180.8053   $180.81   $180.80   $180.80  
  Purchase #3 $179.64   3.652%   $186.2005   $186.20   $186.20   $186.20  
  Purchase #4 $195.93   12.126%   $219.6885   $219.69   $219.70   $219.70  
  Purchase #5 $140.60   11.208%   $156.3584   $156.36   $156.35   $156.40  
  Purchase #6 $147.82   9.788%   $162.2886   $162.29   $162.30   $162.30  
  Purchase #7 $184.67   9.557%   $202.3189   $202.32   $202.30   $202.30  
  7 purchase totals = $1,039.77       $1,129.1305   $1,129.14   $1,129.10   $1,129.20