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 $235.57   12.476%   $264.9597   $264.96   $264.95   $265.00  
  Purchase #2 $226.43   10.308%   $249.7704   $249.77   $249.75   $249.80  
  Purchase #3 $110.90   8.792%   $120.6503   $120.65   $120.65   $120.70  
  Purchase #4 $230.29   3.107%   $237.4451   $237.45   $237.45   $237.40  
  Purchase #5 $149.31   7.391%   $160.3455   $160.35   $160.35   $160.30  
  Purchase #6 $11.52   9.130%   $12.5718   $12.57   $12.55   $12.60  
  Purchase #7 $48.57   11.750%   $54.277   $54.28   $54.30   $54.30  
  7 purchase totals = $1,012.59       $1,100.0198   $1,100.03   $1,100.00   $1,100.10