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 $250.58   12.386%   $281.6168   $281.62   $281.60   $281.60  
  Purchase #2 $121.33   11.562%   $135.3582   $135.36   $135.35   $135.40  
  Purchase #3 $100.31   6.562%   $106.8923   $106.89   $106.90   $106.90  
  Purchase #4 $60.54   11.790%   $67.6777   $67.68   $67.70   $67.70  
  Purchase #5 $206.28   6.623%   $219.9419   $219.94   $219.95   $219.90  
  Purchase #6 $23.85   7.413%   $25.618   $25.62   $25.60   $25.60  
  Purchase #7 $23.70   4.807%   $24.8393   $24.84   $24.85   $24.80  
  7 purchase totals = $786.59       $861.9442   $861.95   $861.95   $861.90