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 $121.57   11.261%   $135.26   $135.26   $135.25   $135.30  
  Purchase #2 $184.74   7.535%   $198.6602   $198.66   $198.65   $198.70  
  Purchase #3 $68.75   3.263%   $70.9933   $70.99   $71.00   $71.00  
  Purchase #4 $82.35   11.493%   $91.8145   $91.81   $91.80   $91.80  
  Purchase #5 $101.39   12.178%   $113.7373   $113.74   $113.75   $113.70  
  Purchase #6 $26.67   9.187%   $29.1202   $29.12   $29.10   $29.10  
  Purchase #7 $144.63   5.163%   $152.0972   $152.10   $152.10   $152.10  
  7 purchase totals = $730.10       $791.6827   $791.68   $791.65   $791.70