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 $131.96   5.215%   $138.8417   $138.84   $138.85   $138.80  
  Purchase #2 $157.47   8.525%   $170.8943   $170.89   $170.90   $170.90  
  Purchase #3 $129.35   12.145%   $145.0596   $145.06   $145.05   $145.10  
  Purchase #4 $181.69   8.625%   $197.3608   $197.36   $197.35   $197.40  
  Purchase #5 $178.73   5.790%   $189.0785   $189.08   $189.10   $189.10  
  Purchase #6 $130.80   3.868%   $135.8593   $135.86   $135.85   $135.90  
  Purchase #7 $96.44   8.190%   $104.3384   $104.34   $104.35   $104.30  
  7 purchase totals = $1,006.44       $1,081.4326   $1,081.43   $1,081.45   $1,081.50