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 $225.57   11.237%   $250.9173   $250.92   $250.90   $250.90  
  Purchase #2 $67.84   4.378%   $70.81   $70.81   $70.80   $70.80  
  Purchase #3 $210.45   4.412%   $219.7351   $219.74   $219.75   $219.70  
  Purchase #4 $20.73   9.592%   $22.7184   $22.72   $22.70   $22.70  
  Purchase #5 $21.59   3.753%   $22.4003   $22.40   $22.40   $22.40  
  Purchase #6 $142.65   12.618%   $160.6496   $160.65   $160.65   $160.60  
  Purchase #7 $128.26   10.715%   $142.0031   $142.00   $142.00   $142.00  
  7 purchase totals = $817.09       $889.2338   $889.24   $889.20   $889.10