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 $15.93   13.489%   $18.0788   $18.08   $18.10   $18.10  
  Purchase #2 $6.40   9.521%   $7.0093   $7.01   $7.00   $7.00  
  Purchase #3 $65.49   13.345%   $74.2296   $74.23   $74.25   $74.20  
  Purchase #4 $29.70   11.495%   $33.114   $33.11   $33.10   $33.10  
  Purchase #5 $126.57   9.341%   $138.3929   $138.39   $138.40   $138.40  
  Purchase #6 $209.27   4.454%   $218.5909   $218.59   $218.60   $218.60  
  Purchase #7 $157.60   4.483%   $164.6652   $164.67   $164.65   $164.70  
  7 purchase totals = $610.96       $654.0807   $654.08   $654.10   $654.10